A couple of bits of advice.....just my $0.02
Don't bother full floating that rear axle. If you're only going to run a v6 that rear dana 44 (or is it a M20 w/ factory 1 piece alxes) should hold up just fine. I'm assuming the donor had a quadracrap, so that Dana 18 xfer should work fine. Leave the 6 lug bolt pattern, its the same as toyota and chevy, read cheap wheels. That light weight M38A1 with a six banger would not get much benifit much from rear disks. Grab the free drums and save some cash for new gears/lockers. Leave the axles full width (unless you want a street vehicle). This will save you much cash and a bunch of time (worth money). Move the spring pearches on the rear axle and move the spring mounts up front to outside of the frame. Moving the spring mounts sounds scarry, but its way easier than cutting down an axle. Just make some triangle gussets.
As far as the engine, I'd run run the evenfire 3.8L. The 225 is a good engine but the 3.8L evenfire is easier to trouble shoot than the odd fire. Plus you can find plenty of 3.8L's in the junk yard. Many had factory HEI. Evenfire engines go: bump,bump,bump,bump oddfires go: thump,thump----thump,thump. That's because the time between the cylinder firing is not evenly spaced.
So long as the SM435 has the butterfly pattern the bearing retainer is the only worry. Check the input splines and pilot bushing though. A clutch disk change may be in order. Also, the buick manual bellhousings are hard to come by, so don't screw it up.
As for order, do the axles/spring mounts first, that way the jeep is somewhat mobile. Gears can wait until later. Tranny next, set it up with the xfer case adapter. Bellhousing next. Bolt any of the engines blocks to the bellhousing/tranny/xfercase. Measure, measure, measure, weld motor mounts, measure (grind/reweld if need be). Drill frame rails and bolt up crossmember. Start messing with engine on the bench. Order driveshafts. Send out block to be machined. Go nuts with wiring for a couple of days. Get engine block back from machine shop. Assemble engine. Install clutch. Swap good engine with dead block in jeep. Continue going nut with wiring. Add oil to all gear boxes. Start engine. Install driveshafts. test drive. Come home with huge f**king smile. Drive jeep to mechanic to install 4.xx gears. Go nuts trying to find something to do while jeep isn't home. Reorganize garage. Find missing tool behind work bench. Go pick up jeep. Come home with huge f**king smile. Thank wife/girlfriend for not getting pissed off for spending all free time with jeep and not her for the last three months. Pack for moab. Any questions?
Best of luck on the project!
PS, I started a thread awhile back on the do-it your self rear full-float. http://bbs.off-road.com/wwwthreads//...vc=1#Post42357
My name is Ted, and I'm a Jeep-a-holic.