I think the length of the rods only changes where the pistons ride in the bore, not the stroke, which is determined by the crank offset. Longer rods might raise compression ratio, depending on wrist pin location and other piston factors.
It'll be a 406 or bigger, with mildly ported stock heads and a mild cam, and we're thinking about an EFI.
Thanks again for the input.

cm

If you can't fix it with a hammer and screwdriver, you need a bigger hammer.
1965 Chevy C10