Cool guitar! I'm also planning to put a Bigsby on my 1963 H77. The Harmony H78 (same guitar but with a factory vibrato) came with a Bigsby model B3:
While the B3 would definitely be the safe route, I don't like the fact that the Bigsby handle does not extend much past the bridge--I'm used to Gretsches, where the handle extends a fair amount past the bridge, making it easy to grab while playing. I've been measuring my 6118-LTV and the Harmony this morning, and I'm thinking that I might gamble on installing a model B6. The potential downside is that there may not be enough string break angle. I may pull the B6 off my Gretsch and try it on the Harmony before ordering one.
I hope that helps. Best of luck, and I'd love to see a photo of the completed project.