I know this is an old topic, but it might help inform someone. I have a 67RS, 327/275, all standard. I have the engine stripped down to the intake and I am about to pull it.I plan to use an HEI on a 350. I decided to try the HEI for fit, and , with the cap/coil off, it will not allow true vertival enough to slide completely into the block. With the cap on, it wouldn't even start to go in. I took the time to measure the standard distributor and the HEI (caps on) and the HEI is almost exactly 1 inch larger in diameter and about 1 3/4 inches taller. If the firewall were 1/4 inch deeper, I could slip the HEI in w/o cap and then install the cap, with little clearance. I have decided for a cleaner approach and cut and section the firewall. I am beginning to think there are minute differences between cars that aren't documented anywhere. But there is no way an HEI will slip in to the existing manifold/block without moving something.