Get a 85-up block just to be safe. Internals on the 85-92.5 are all the same with the exception that the 86 had flat tops. Halfway through 92 they did go to hyper pistons but it really doesn't matter as long as your tune is a good one. Plenty of 93-95 cars and cars with explorer bottom ends on boost/spray with no issues.
However what exactly are your plans? I would be MUCH more interested (if I were you) about getting a 94-up 351 block out of a pickup or Econoline van if you are planning on doing any sort of forged internals or a serious pushrod buildup. A 351 stroker is better in every possible way, does not cost hardly anything more and its not just better by a little bit, its like having the choice of sleeping with Brooke Shields in the 80s or Brooke Shields now.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.