Howdy, I just stumbled across this site and saw this thread so thought I would introduce myself here.
I have a 90 Astro converted conversion van, meaning, I have an Astro coversion van, (by Gladiator), that was coverted to a 350 V8. You can read all about how I did it, here:
In answer to some of your statements and or questions: The Astro has always been a subframe vehicle. Just think of a 1st or 2nd gen Camaro that turned into a refridgerator. Same basic setup.
You cannot use the factory engine mounts, as the sbc is 3 inches longer than the 4.3 v6. You can use the factory frame mounts, and the transmission stays in its original location. The only cutting you have to do is to the fan shroud to clear the accessories.
You cannot pull then engine out from inside the van. It has to go out the front or drop the subframe. For the 96 and newer models, they don't recomend cutting the radiator support and bringing it out the front. They say it is easier to drop the subframe. There are still alot of debates about which way is easier.
The AWD astros have torsion bars suspensions, while 2wd have front coils. The front drive axle is the same as the S10, the transfer case is the same used in the Syclones, and the wheels on both 2wd and AWD are of the full size pickup/suburban 2wd's. The whole enging and drivetrain sit off to the right hand side of center, as does the differential. The Astro has one of the smallest differentials, which makes that their weak link.
I have been driving my van with the 350 for about 5 years now, and have had no major problems other than rebuilding the transmission. It runs on the computer, is TBI injected, and the cruisecontrol and A/C work.
I found that the original trans was in bad shape when I got the van. (I bought it with a blown up 4.3 v6.) The original trans soon succombed to the 350.
I average about 21mpg if I keep my foot out of it and can tow a 5.500 lb trailer with no problems, and it has more interior room than a suburban.
Hope this answers some of your questions about Astrovans. I am currently building a 350/5spd powered 85 Mazda RX7. I love Hot Rodding