I recently installed a Summit Ready to Run vacuum advance small cap distributor in my 69 Z28 because the large cap HEI didn't fit, and I didn't want to bend the firewall. Even with that the 2 front spark plug wires (8.5 mm) were a tight fit with the air cleaner on but still worked. I ordered their coil, (also had a big one previously) not too big and works great. Photo attached.