That engine/ Mustang has enough value that screwing w/ it could be a mistake. Boss 302 stuff is relatively rare.
I don't claim to be an expert, I have only played w/ a few Boss 302's and the last one was 4 years ago for a restoration project.
Never did a stroker w/ a Boss head but have done phony Boss and 351W w/ Cleveland heads. The head can move a lot of air so stroke will help. The block should accept 3.400" without brain damage, all it will need is some minor work at the bottom of the bores for rod bolt clearance. 3.250" may need no clearance at all. The key will be the piston, there aren't a ton of them out there for an 8.2" deck w/ a Cleveland type head.
You can also consider port plates that reduce the port volume some. Also lighten reciprocating parts so you can buzz it quickly. Used to do both those and (for the time) aggressive solid cams, then wind 'em up, 8500 9K wasn't out of line. If you stay stock stroke, keep in mind that the Boss con rod is longer than the standard 302 rod 5.155" vs. 5.090" so you can't interchange them. Also the '69 crank is crossdrilled steel and the '70 crank isn't crossdrilled.
Again, original Boss parts are pretty rare and worth some coin, might be better to lift the engine out and pack it up. Build a phony to stuff in it.
Good luck