Whats the widest size rim i can fit in the back on my 64 impala ss? Will a 22x 10 fit ?

10 should fit just fine....depending on where your exhaust is sitting. I had to put spacers on my wheels because the exhaust was tucked in really close. If you have lowered it dramatically that would make a difference also.

I was wanting to lower it some. What kinda difference is it going to make?

to me with a 22" or 24" rim tucked or the car just dropped a lil lower it rides smoother, if anything it improves performance, my boy has a '74 impala on 22"x10" twisted vista rims with 3" turndowns an it jus glides from 100 - 140mph, it eliminates a lil of the bounce in my opinion, plus clears exhaust an frame, have u tried 22's on your car before?

I havent put anything on yet,Just wanted to see whats the widest that can go back there before i buy anything

in my opinion, i think they should fit with no problemz, but r u gonna modifiy rearend? shorten? 9"? if not get fitted first.......
i know 26x10 gave us problems an we had to lift, but 26x9.5 was a better fit, but a lil hard to find a rim that we like...

Thanks