@LGriffin; I must respectfully disagree! The bore angle in the block is the same, therefore the clearance is the same. I had zero problems dropping a 1976 HEI from a 250 into a friends 1966 292 in his work truck. He got the distributor across the counter at a local salvage yard for $20 obviously used. The vacuum advance was marked "10", which I took to mean 10 degrees advance.
We hosed it off with canned carb cleaner and dropped it in. I changed the wire and set the timing at 8 degrees and we went for a test drive. Everything was fine and the engine was smoother. It is possible that you have the side covers reversed front to rear or early covers as the front cover is clearanced for the big cap after 1974. All I know is I had NO trouble.
Best wishes,
Les
