What do you plan to do with the car? There aren't a lot of good non drag radial tires available in 15" these days. 17 or 18 is where most of the current tire development is.
15 and 16, you can get some good tires, but they're limited in size.
With 17's I'm running 275 rear and 245 front on my 67. The backspacing isn't ideal, so there's a bit of rubbing in the rear over severe bumps, and none in the front at the current height (but I might cut the coils in the front to drop it another 1/2" and then it might rub over severe bumps). If I moved the front and rears in 1/4" it'd have 0 issues.
225 front and 245 rear will fit with plenty of space, the wheel wells are a bit tighter on the 65/66 though, so going larger than 225/245 might be problematic unless you have really good suspension and custom backspacing.
Who cares how much horsepower it has, all that matters is how fast it goes!
Untested 331, lots of suspension, chewing up corners.