Not sure if I'm in the right place but here goes

If you just slap common 318 heads on there, they will lower the already low compression dramatically. You need the small chamber "heart shaped" heads (302? casting?)

Also, a replacement transmission........if automatic.......will have to be the older 68/ earlier with the smaller converter bore. "It might be?" the transmission mount on those hangs from above, I cannot remember. That means the tail housing is specific to those old vans