I just did the reverse, as an aside
spring perches line up fine. I ordered a 2" (1 per side) narrower crate rear from Currie, basically, a '65-66 spec, for my '70 convertible. A little flattening of the inner wheelhouse was required, and grinding down the outer portion of the bump stop. Even with that, I still had to add a 3/8" spacer to eliminate rub, but I'm not shy about doing a bit of mini-tubbing/inner wheelhouse mods if I go to wider tires on my 17x9, 5.5bs wheels with 275 rubber.
Are you going to flare the outer wheelhouse? If you go to a decent sided tire to handle the extra power, you might need to . . .