I am thinking that a 275/40/18 (vette tire) on the back
and 245/40/18 on the front.

or maybe the 245's all the way around.

I am looking at the drop spindles in front and the mono leafs are headed out too. Four wheel disc's will be in place also.

How many guys are able to get 18's to fit on 67's or 68's?

Let me know if this is just a pipe dream.

wheel diameter is not an issue here.... overal tire diameter is what counts, along with overal tire width, and rim width and backspace.

The sizes mentioned can fit if the wheel backspace is perfect. Your 245 on the front is on the big side of what fits and backspace will be especially critical