I replaced the upper intake manifold and gasket with the updated version from Napa to resolve the common oil and coolant leak issue on the 3800 (99 Lesabre). I also replaced valve cover gaskets and cracked coolant elbows to left of intake. 4 months later its worse than ever. Oil is pooled at the base of intake, between it and the valve covers, and I can see swirls of green coolant mixed in. Fresh looking coolant is all over the engine under the throttle body and a layer of brown sludge has formed under radiator cap. Clearly the fluids are mixing. I had pressure tested cylinders before swapping intake to check for bad head gasket and results were good, also car is a daily driver, given its only a 15 minute trip to or from work so I doubt its the head. I did not replace LOWER intake, and I admit I did not torque intake to spec but I have a feel for how tight it should be and I doubt it would be this bad. Please help, my experience is limited and this car is frustrating me...

only answer is to do it over this time would replace upper and lower with the Delco
aluminum gaskets on bottom and maybe get new upper manifold as now it could be warped the bolts are only torqued to Lower intake manifold bolt 11 ft lbs
Upper intake manifold bolt 89 in. lbs
This time use a torque wrench with the right sequence
be care full with the radiator drain and even with lower hose off
still lots of water in the block
Enough were you can see it with lower intake off in the water jackets
you have to remove a lower timing cover bolt on each side to get it to drain more
Or if you want to total drain the block you have to take out the knock sensors
and the one next to transmission not fun takes a 7/8 12 pont crow foot on a foot long extension to get it out and 12 inch long needle nose to fish it out

^ ^
Agreed, I'm sure if it'* the original lower intake gaskets they are junk. Time to replace the lower intake gaskets.

x3, on the ones i have done the plastic gaskets on the lim were all blown out. fel pro makes a good aluminum lower gasket and dorman upper set, can be had from rock auto for about a hundred bucks. then get an inch lb torque wrench and do it in steps for both, super important to do the right pattern and steps on plastic and aluminum

There was a past thread with all the Felpro and Dorman recommended part #'* for this upper/lower project... anyone happen to know where that info is (or have it handy)? Also is it fairly common that if the gaskets are leaking that the intake(*) will be cracked/warped somewhere need to be replaced too?

(My '00 LeSabre is probably going to need this procedure, and I only want to do it once but don't want to spend a ton of money on a car only worth $1200 anyway).

The main issue in my experience has always been with the lower intake manifold gaskets, those should have been replaced as they almost always eventually fail.
You are going to need to remove the UIM and inspect it to see if it is still reusable, then remove the LIM and replace that gasket with Felpro MS 98014T.
For those coolant elbows, if you used plastic ones I highly recommend going with the aluminum ones Dorman 47065HP.
When you get to the LIM be sure to replace the EGR Stove Pipe, you should have got one if you got the Dorman UIM Kit.
Make sure you flush your coolant out, add fresh coolant, and bleed the air out, and do an oil change, then when done drive it a for a few days, then change the oil again, considering your findings you may want to replace the coolant again when you do the oil change the second time.

Let me attach some pics for you, that way you can see the UIM and LIM torque sequences.

Thanks for those numbers, exactly what I was looking for! What'* a realistic number of shop hours for the UIM and LIM work? I was quoted 6-7 hours at $70/hour by a nearby garage mechanic I found, which seems pretty steep to me. Parts not included. I do not have the time, skills, or tools to attempt the job myself... but don't want to get raked over the coals on cost either.

Very welcome.
Somewhere around 6 hours for both the UIM/LIM sounds about right, just be sure they are draining the oil, coolant, and refilling them.
With the right info provided the job is not too hard to do, when I did mine I had never done an intake job before, and with a little help from forum people, and a video or two, I had all the info I needed to do the job, if you are slightly mechanically inclined it is not too bad of a job.
A shop is going to run you around 550.00 - 600.00 with parts included, as they have to pretty much double the price of the parts to warranty replacing them if they prematurely fail.
With a few basic tools, and a inch lb, and ft lb torque wrench, and basic mechanic skills, you can do the job yourself for a fraction of that cost, but if you are really uncomfortable trying to do the job just get a mechanic to do it.
For the LIM Gasket Kit I payed like 54.00, the Aluminum Coolant elbows are around 9.00, I replaced my upper intake with a new redesigned Dorman one with enforced baffles, it was around 73.00 for the upper intake/kit, so for those 3 main parts I spent around 137.00, I got my parts from Amazon, if anybody is buying the parts themselves I'd go with Amazon, or RockAuto.
Of course that 137.00 does not include the new thermostat I decided to put in while I was at it, nor the oil/oil filter, or coolant price.

That might describe me. I mean I've done hundreds (okay maybe dozens) of oil changes over the years and several slightly harder jobs like full tune-ups and replacing a radiator and starter... all bolt off / bolt on stuff. I've paid for motor mounts and a timing belt & water pump combo replacement on my '07 Chrysler Pacifica, because those were above my comfort level. My main issue is TIME. I'm sure a 6-hour job for me would end up taking 10 hours over 2 days, because stuff ALWAYS seems to come up... a tool I don't have, a bolt that won't come off, something I can't reach to remove, etc. With work/family/school I just don't have that kind of free time to take my car apart and HOPE it still works when I put it back together! (haha) Even though I really like fixing stuff myself and learning + saving money. I'm hesitant on self-repairs when it gets above a couple hours.

Well Scotty, if the land of Lincon means Lincon Ca I would give you a smoking deal on the job for ya!