New 904 converter doesn't fit.... uh-oh...

I am chasing an engine-speed related vibration on my '71 Dart, 318/904.

Took off accessory belts, no change. Installed new balancer, no change. Dropped pan to inspect rotating assembly, nothing weird there, replaced original '71 rod bearings. Flexplate good. Figured it's gotta be a converter problem, though there are no weight on the converter there is a small crack in the starter ring.

Bought a used (400 street miles) 904 Boss Hog converter, Summit part number 53022 that had previously been installed on a neutral-balance LA stroker. The transmission in the Dart is actually a '72, don't know if that matters or not.

Dropped the trans today and compared the converters. The Boss Hog has a starter ring diameter of about 12-1/2 inches compared to 13-1/4 inches on the one I pulled out. The snout on the Boss Hog is also a little over 1/4" smaller at just under 1-1/2 inches to the old converter's 1-3/4.

I thought all post-'68 904s were the same?