8" too short in a 67 fastback

I have a 67 mustang with a 8" rear end that seems to be out of an early 66 mustang it measures 52" flange to flange I've been told that it should measure 54" is this correct

Was your car originally a 6 banger? maybe the 8" was swapped in from an early V8 car. It should be 54" you're correct

You can still use it the perches will be in the correct location.

If you are ordering custom wheels for the rear simply subtract 1 inch of back spacing...so a 17x8 that you would normally order with 4.75 would need to have 3.75 bs. The other option is to just bolt on some 1 inch adapters to make up the difference. I am intentionally putting a 65 width rear in my 67 so that I can run a much deeper dish rim in the rear.

Yeah, I just put a granada rear in my 67 and it's about 3/4" narrower on each side. I'm going to get some spacers for my wheels since I already had them!!