so im getting a rebuild from the lower intake manifold on up. New head and intake mnfld gaskets...however i had a couple of questions..How much mileage in theory would this take off???i know there isnt an exact number..the upper intake is plastic (im sure u know) so i deffinatley will replace that...should i get metal head gaskets and lower intake gaskets?....i would also like a couple of cheapo preformance modds...maybe get it bored out a little ? what do u think?

I'm not sure what exactly it is that your talking about mileage. Are you wondering how many more miles you can get out of that engine after doing the top end rebuild?

Stock head gaskets should be fine if you are staying pretty close to stock. With the lower intake gaskets, get the aluminum framed gaskets.

Boring over to 4.0 is not exactly a "cheapo mod". This is a very in depth procedure that involves removing the engine, stripping it down to the block, and taking the block to a machine shop. After its "bored" over, you will need bigger pistons, connecting rods etc. Then you will need to have the PCM reprogrammed for the bigger displacement. Surely, not a "cheapo mod".
Cheap simple mods for either the L36 or L67 would consist of a cold air intake, even fender wall intake, better wires, plugs etc. Then it starts to get expensive for the L67 when you start upgrading the supercharger pulley, port and polish intake/exhaust etc.

When i said millage i was referring to how much "wear and tear" this top end rebuild would take off the engine. I hear rebuilds take quite a bit of miles off an engine.... but Maybe ill start with boreing the throttle body and replaceing the ubend pipe with a wider strait threw pipe from zzp. Any other ideasother then plugs and wires?

If your just replacing gaskets from the head level on up, your not reducing/removing any wear and tear. Just prolonging the life of the engine.

If you have already been perusing ZZP, then I'm sure you have already seen most of the common mods for the L36.

What are some things i can do to take away some wear and tear? I have a brand new water pump already..im thinking valve train replacement...but i dont know too much about rebuilds...my main thing is to reduce wear and tear...

Take away wear and tear? You are looking at a complete tear down from top to bottom for a COMPLETE overhaul. Rings, pistons, bearings, EVERYTHING. The parts and labor alone would probably cost well more than a GM certified reman engine.

To reduce wear and tear, change oil/filter on a normal basis, upgrade to synthetic, and don't drive it like NASCAR. I, along with a couple others have destroyed a few 3800'*. I probably have the most deaths. Billboost37 is notorious for transmissions along with Danthurs.

I had a top end rebuild (and then some) done on my Bonneville during my cam install. The short, obvious answer here: to "undo" wear and tear, replace any part that spins, moves, heats up, or has fluid touching it. But if you did that, you'd be under the hood for days.

You're lucky enough to have the 3800 engine. The 3400 likes to blow head gaskets, and the 3100 has its own problems. With proper care and feeding, you'll easily get 200,000 miles minimum out of this engine.

Let me outline the stuff that I think is important, more or less in the order it comes out/off the engine. I'll point out where you can mod things, but none of them are really that cheap. Your opinions may vary.

• Coolant: Is your coolant green? Replace it with green when you're done. If it'* red, orange, or black, flush it with water until it drains clear, then pour in green.
  
• Coolant hoses (depending on how you drain it): any bulged or cracking hoses should be replaced. Nothing to mod here.
  
• Throttle body: Remove the throttle body and the MAF sensor, then grab a can of carb cleaner and go to town until the insides look clean and shiny. Repeat for the IAC valve. There are a few mods here (like polishing), but on a GT, they're of debatable usefulness.
  
• Fuel injectors: on a GT, you're going to need a LOT of mods for these to be a hindrance. They rarely need to be cleaned by hand, so just lube the O-rings when you put them back on. Replace any bad-looking ones.
  
• Fuel pressure regulator: do you smell fuel in the vacuum line? If so, replace the regulator, otherwise, keep it. No mods here.
  
• Vacuum lines: If there are any broken or cracked lines, or brittle rubber, go to the auto parts store and roll your own replacement.
  
• Upper intake manifold: 3800 Design Flaw #1. If you don't know how many miles are on this, it'* best to replace it. Try to get the newer model that has a metal sheath around the EGR stovepipe area. Buy new gaskets if not included. For $300, you can drop in the ZZP runners (I forget their name) and adds horsepower, at the expense of a little torque. This might be one of the bigger mods for a GT. I know I'm putting that on my list.
  
• Lower intake manifold: The gaskets are 3800 Design Flaw #2. As Mike said, the new metal-framed ones are mandatory if you want this engine to live a long, happy life. For a tiny performance boost, try to port and polish the air ports (search; I don't know how this is done).

You won't be going deep enough to hit the head gaskets, but that'* fine because you have a better chance of winning the lottery than breaking one of these on a non-supercharged 3800.

With the LIM out, you could then pull the valve covers and rockers off to check out the lifters and pushrods, but that would be a lot more work, and I'm clueless on how to do that. There'* another great mod here (E-R rockers) if you choose that route.

Also, because you opened the engine up that far, change the oil and filter at least once, just in case any coolant or dust got in the crankcase.

Well said Colin!!

Probably the best answer ive ever recieved on here...the coolant thing was always a queztion i had...the original stuff is crap and carrosive...im definitely changeing lower intake gaskets, head gaskets and upper intake mnfld gastet aswell as coolant switch..thank u bro...

so i have the money and im takeing it in, will do all of the main things u all advised me to do...i have a little extra money ...not enough to buy a new tranny but maybe enough to take a look at it and trouble shoot it...the trouble is i notice that it takes a while for my tranny to build up enough pressure to shift...i can hear it and feel it...its a high pressure sort of sound...then it shifts after holding the throttle down for a while...it shifts firmly like new...just takes a while to buld up the pressure to get there....im thinking a new pressure solenoid...this being the last thing i had a question about what do u think it is ??