If it's aluminum roof, and I was hoping it was (at least NOT rubber sheet), then cleaning it up and applying Eternabond Tape to the seams is most likely the best option.
One of the excuses manufacturers used for going from expensive aluminum to cheap rubber was expansion/contraction of the aluminum leading to leaks at the seams. That was before Eternabond! Nowadays you can apply EB and seal it for good. It'll allow stretch and shrink. Get the job good and clean. Lowest EB price we've seen was Best Materials. You can call EB and get loads of tips over the phone.
The other excuse was that holes pitted through it. Probably true, inspect, keep clean. I saw one where the TV antenna was bent down and cut through the aluminum causing a pesky leak. Owner always worked on it with antenna down and never noticed.
You're golden with 16" rims. The grille looked late model enough that I thought it would not have the former 16.5" which are a larger diameter, but use a lower profile tire. They're obsolete and only a few tire companies offer the 16.5 tires, so good for you.
We had an older Dodge Van, a '71 which offered only the 318 engine. After 100,000 miles we replaced it with a 360. TONS of difference. 360 is plenty, don't let somebody talk you into a big block like 383 or 440. Van engine swaps are a buttload of work, but what's worse the BB takes a different front crossmember than the SB. You'd have to take the whole suspension off to change it, if you could find it.
