Will the following combination fit on a 71 Chevelle? Front: 15"x8" with 3 7/8" BS with 255/60/15 TA's and Rear: 15"x8" with 3 7/8" BS with 275/60/15 TA's. Based on previous postings it looks like 4.5" BS is best but the wheels I want only come with 3 7/8" BS. Will they fit?

My guess would be that combo would not work very well,,on either end.

nope .. fronts will hit inter fenderwell on bumps & turns.. 245x60's on 8" rims with 4"b/s rub on my car fronts ... rear is no problem I've run 275x60's on 8" 4 b/s best front I've found is 15x7 4" b/s with 245's this is on a 68 El Camino

Glad I asked. Would the following combination be better? Front: 245 60 15" on a 7" rim with 3 7/8" BS and Rear: 265 60 15" on a 8" rim with 3 7/8" BS. I was told a two size change looks good front to rear. I thought the 255/275 combination would work because the car currently has 245's all the way around on stock rally rims and it looks like a ton of room. I don't know if it matters but the rear is raised 2" from stock.

That should be fine for the front, but your back tires are still going to be really close. You'll be OK if you're prepared to roll the inner lip of the quarterpanel up a little if necessary. If it were me, I'd look for a wheel with about 1/2" more backspace.

Thanks for the replies. Based on the searches I did I thought the recommendation would be to go to 4.5" or more backspacing. I am kind of set on putting Keystones on the car (saving the original rally wheels) because that is what I had on the 72 I had in high school. This is a father son project and he likes the pictures of dad's old car with Keystones. I have only been able to find them with 3 7/8" BS or Reverse BS.