Is there a reason that is HAS to be 70's?
3rd gen Ford and Chevy/GMC used the same body style up into the 90s (75-91 for Ford, 95 for GM...96 for a G30). Dodge was 71-78, then started the new body in 79 and carried it until 1993.
it is relatively simple to make them resemble a 70's van with grill swaps (and taillights on GM) to where the differences are subtle, but you can often find lower mileages, AC, less Rust, and even Fuel Injection with Overdrive. What I am saying is, why limit yourself? if its just for the 'cool' factor, non-van people wont even know the difference.