1970 duster slant six 8.25 rear swap

I don't think you're in danger of "blowing up" the 7 1/4 unless you're planning to build your slant to some extreme. Most people who advise to swap out the 7 1/4 do so because they are smaller and don't hold up as well to higher power levels/racing.

Unless you're gonna be running boost or some other way of making much larger power numbers, you'll be fine.

Now if you're planning a V8 swap for the future but you have an 8.25 right now, I'd say go ahead and get it done, as that's one less thing you'll have to worry about down the road

EDIT: Its worth noting that if you're dead set on doing the swap anyway, Dartman 65's info is what you're looking for.