In general, dual field alternator in a single field application with single field VR is easy... just ground one field terminal.

The other way, single field alternator in dual field application, not so much.

Easiest to stop at another AP store... they should have some flavor of dual field in stock. Anything from a '70 roundback dual field rated 50 amps up to a late square back rated 78 amps should get her home. Also a good idea to pick up a regulator in case the current one is fried.