thats a lot of work there bud. just as easy to do a v 8 swap as it is to put a big 6 in one of the older vans. lots of stuff has to be changed, including part of the floor for clearance. you dont have to do that to swap in a v 8!
if you just have to do it, you are gonna need a 65-67 that came with the big 6 for parts. do lotsa cutting and welding cause nothing from the little 6 will work on the big 6.