Tim,
I would not have expected it but you are right on the money...the tires available for 15 inch rims have a bigger overall diameter than the 16 inch offerings. Tires for the 16 inch are cheaper too.
I referenced Coker tire's website. They are near me (in fact I was there yesterday and went through their museum)so that's probably where I will get the tires.
So...to answer my own question. 16 inch tires are cheaper and have more widths available, so that's what I will go with.
Joe
P.S. When I bought my engine, the guy had (7) 15 inch wheels setting there. I called on a parts truck in Knoxville last week and yep, 15 inch wheels, craigslist...15 again. It just seems like I run into the 15 inchers more readily.
