on a 17x10 w/5.5 BS would fit in the rear of a 71' w/o any mods? Car is getting about 2" cut off the rear spring. I know a 275 would fit but am wondering if the extra .4 -.5 inches to step up to a 285 going to cause me problems and if so could the BS be increased alittle? Looks like I'm buying tires over the internet so there will be no test fitting.

Shouldn't be a problem. I have 285/40/17' on My 69, and I believe 71' have a bit more room. I have the same backspacing and rim size as well.