tire size? up or down

1 inch = 26.5 mm so, 10.5 wide x26.5 = 278... go with 275x60 R15s. I can measure a set later this morning (on the back of my 69 barracuda right now). 60 means sidewall is 60% of treadwidth (275mm) or 165mm. so, 2 sidewalls is about 12.4 inches, 15" for the rim, you are about 27.4" tall. If you want shorter, go with 275-50-R15 That will shorten the overall by about 2" and end up being about 25.4" tall. I can also measure a set of 295-50-R15 for you.