The 350 Chevy was used from 1967 to 2005 in various vehicles. It is still being used by Mercury Marine. Sure, there are subtle differences between them all, but I guess a person could say that is offset by the fact that parts from other small block chevy engines also swap over.
You should have a 2 piece rear main seal in that engine. That means stuff from 1967 to 1985 will fit it. Even if it had a 1 piece rear main everything but the crank would swap. The only 350's that lack a lot of interchangeability of parts might be the LT and LS series as well as the L31 Vortec motors. Even then...A LOT of stuff is the same.
The only unique thing on that van will be the short water pump setup. That's not terribly unique, but it is not what you would find commonly on say a truck 350. Does it matter? Nope! You have all the correct brackets and a short water pump is no more expensive than a long water pump.
Assuming it is a Turbo 400 tranny it was used from 1964 until atleast 1992 and possibly later depending on whom you ask. If it is a 700R4 you're looking at 1984-1992 with a lot of interchangeability with the 4L60E still used today.
Your rear axle should be either a 14 bolt or a Dana 70 as I stated before. For some interesting reading check out Billa Vista's 14 bolt bible. I think you will quickly understand how great that axle is as well. http://pirate4x4.com/tech/billavista...ble/index.html
Don't be scared. Give in to it. It truly is simplicity.