Are you going for the 6600 CPU with PPU or just the CPU itself? The 6400 CPU, on the other hand was one of the initial proof of concept targets when Neil Lincoln at (then) ADL and later ETA was playing with his "box of Chiclets" IC concept. To the best of my knowledge, a 6600 was not attempted. Even then (in the 1970s) the small footprint was amazing.
--Chuck