Currently using 235/60/15 all around. Want to get fatter in back, but not much taller. Will a 295/50/15 fit a 68 Roadrunner without any modification or issues? I do not want to use air shocks or anything like that. If there is enough room back there for a 295/50/15, specifically what size rim and backspacing would you use? If you know, it will save me a lot of measuring and searching and wondering. Thanks.

Yes on a 10" rim with 4.5 back spacing they are only about 26" tall. ran them on an 8" rim also with 3.5 back spacing also.

I think I had 15x10 rims with 5.5bs. Was very close to the springs, but prefer that than hitting the quarter lips.

You will need a 10" wheel with a 5.5 backspace and they will clear. You do need air shock but if you have a week rear suspension you may experiance some rubbing with a full load in the car. I have run the same wheel with 305-50-15 tires for many years and have never had any issues. I currently have 325-50-15 on my wheels and with the quarter panel lip rolled are able to stuff them under the car. If you are looking for some meat up front you can use a 15x8 wheel with a 4.5 backspace and a 265-50-15 tire with no issues.