The 70 back axle is 1" wider per side than the pre-1967 Mustangs. also, you will have to change the universal joint to all small cups, and the brake line will not mate.
On the plus side, you now have the ability to put in bigger rear brakes.
9" in a 70 convertible? What engine? only the 351C & 428 rags got the 9"....
I am not sure on the upper & lower ball joints, There may be a difference in dismeter & taper. The toe rod ends do not match at aall, and are tough to match up. but can be done. Hopefully someone else her can answer, and do a search. also, you may have a problem with factory wheels and hup diameter.
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 . . .