small block crank

Yes, it is likely a 273 crank. Early 273's had a smaller register for the converter hub. I found out the hard was a long time ago. I don't recall what year around the mid-60's they went to the common, larger size.

I had that issue a long time ago. A friend ground it out with a die grinder because I was at work and had a race to go to the next day. It may not have been the "right" way to do it, but he was very careful, grinding it out bit-by-bit, and I don't recall any issues.

The alternative is disassembly for a visit to a machine shop or another crank, or a converter with a small snout.