Can you even find FP-sync flashbulbs for use with a focal plane shutter on a Speed Graphic or M-sync bulbs for use with the leaf shutter? If the shutter has a sync terminal, and it has X-sync, I'd use electronic flash if you need it.

Tri-X 400?--Not unless you're using a rollfilm back. In 4x5", you would be using TXP or the old TXT 320, but if you process in Acufine, you can get EI 640, which is what I use when I shoot handheld in available light with my Linhof 4x5". At 640, you can shoot at about 1/30 sec. at f:3.5, which is about what you need for most indoor situations.