Couple clarifications ... The 275/50's are on 15x8 wheels on the back of my '69, not the '66. There's no way they'd fit in my '66.
I know that I could go with a bigger tire than the 225/60's that I'm running on the 15x8's in the front of the '69. The only reason I stuck with the 225/60's is that I wanted to keep the diameter no larger than the years. The 225/60's have a 1/2" smaller diameter than the 275/50's. I would rather have run 235/60's in front, but they would have actually been slightly taller than the 275/50's. One thing to keep in mind with outside diameters of tires, is that a wider weel will actualy reduce the overall diameter of the tire as it spreads it further.
When choosing my tires, I went strictly by the measurments I took with a tape measure, then chose my tires from BF Goodrich's website, based on the width and height of the tires using 15x8 wheels (they tell you the calculation to use for reducing/increasing overall diameter based on wheel width on their web site). Using my measurements with their charts, they came out PERFECT.
The '69 has 620 1" lowered springs in front with the Shelby drop. So far I have not rubbed a single tire ... but then again, so far I have about 1 mile on them

(no, that's not a typo ... ONE mile, because I haven't installed the seat belts yet).