Astro Vans are not a subframe vehicle. If you seen the front clip sitting in the yard it is because somebody cut the frame rails on it. You change the engine from the inside. Believe it or not, it is easier to change by pulling the seats and engine covers and going through the side sliding door with an engine hoist.
There are several V8 Astro conversions out there. They share the same components as alot of the other RWD GM cars/trucks. You must always look at the odd parts if you want something to stay under budget. IE: Most people would go after a Camaro subframe...why not go for a S10,Monte Carlo,Cutlass,etc front clip for 1/3 of the price? They share the same components and setup but one might be slightly wider than the other. I say take some measurements...head to the wrecking yard and start looking for what will fit.