B&M 10235 should work with a cast crank 340/904. The multi-bolt pattern on B&M flexplates cover most OEM & aftermarket converters. It's all in the bolt pattern and bolt hole sizes. If the B&M plate is for an external balanced 340, it won't matter if the converter is a 727 or 904 as long as it bolts to the neutral converter..

* When you disassembled the 340/904, was the flexplate neutral?

* Does the 340 have a "FOR 340 CAST CRANK ONLY" balancer on it now?

* Pull pan to confirm if it's a cast or forged crank. (Even though '73 "should' be cast. (Check parting line on crank.)

* Possible but not probable that it's internally balanced.

* Direct Connection use to have a converter balance kit to add weight to converters on engines that were externally balanced. I believe their race manuals had converter location templates and the weight required to do it from scratch.

* Can you ask the previous owner if he had any vibration issues?