Right; definitely positive ground originally. Oh, and if it still has its original Lucas coil, you'll see SW[itch] and C[ontact]B[reaker] (or points) terminal markings! The CB terminal equals the + terminal Jeff notes.
Is this the Powder Blue convertible that was just on eBay? I'd really like to hear more about this car, as it appears to have some unique original features (which likely are appropriate to its history). Please feel free to send a private message!