Have the opportunity to purchase a 72 Dodge B100 Van with a slant 6,for next to nothing because it needs a tranny(Automatic).

I can also score an entire 1980 D100 Longbed with a bad slant 6 engine but good Torqueflite tranny for dirt cheap.

Is the tranny in the 1980 truck a lockup style?

Will the truck tranny bolt right up to the 1972 slant 6 thats in the van,or are there tailshaft issues to also consider?

It should bolt right up. Look underneith and if the van has a 1 piece shaft & both have a slip yolk in front you should be home free.