I have not done this on a 3800 but I have rebuilt lower ends on other vehicles without pulling the engines. I rebuilt the lower end on my Dodge P/U because of the same symptons. I used stock bearings because as long as the crank journals are not damaged it should be virtually the same diameter. The babbit material of the bearing is very soft (it is tin based) and does not come into contact with the journal because of the hydrodynamic wedge created by the oil.
I pulled the oil pan and took off a connecting rod end cap and journal bearing cap to inspect the bearings. The babbit material was pretty much wiped but the crank was still in good shape. From there I took off all the rest of the caps and to remove the top half of the bearings I used a flat piece of flexible plastic and tapped it up through the top of the rods and journals to spin the top half of the bearings around.
I was able to get a high volume oil pump for the truck but from what was said earlier, that may not be an option of the 3800. The assembly is opposite to disassembly and the only one that is difficult is the front because of the timing chain and the front cover. I simply had a buddy pull down on the balancer (he was over 200lbs) which gave the clearance I needed to tap the bearing in place. I plasti-gauged the assembly to check clearance and all was great.
To start it up I removed the coil wire and cranked until I got some oil pressure (I oiled the bearings during assemble...). I had the truck for about another 100,000kms before giving it back to my father and then it ended up going to my brother. He eventually got rid of it.
Now, this does not fix the camshaft bearings which could be a source of pressure loss. The whole job took me about 4 hrs but that was at the college with a hoist and every tool available.
Either way you look at it, the oil pan has got to come off and from there it is just a couple of minutes to remove an end cap to inspect the bearing.
This is not a beginner job but it is possible and of course does not replace a full engine rebuild.
Just though I would give you something to think about.
1992 Bonneville SSEi
White, grey Leather interior.
Tinted windows and blackouts on headlights.
Custom high flow exhaust, Everything replaceable - DONE
300 000 kms original engine, tranny 2 rebuilds.
Playboy Bunnies Embossed in Tint. Oh Yah!
Ever Seen a Polar Bear in a Snow Storm?