Another vote to stick with a Buick motor. My '55 has a 401 backed by an ST300 but I hope to change that to a 400 next year. This one leaks and slips and I'd love another gear. I also have an open DS and 4 link. I don't think I saw anyone answer your question about a northern California nailhead builder but to me the obvious answer is Matt at Centerville Auto Repair. He's close, well respected but very busy. I think I recently saw him post that he's into 2019 before he can get someone in for a rebuild.
Thanks for the info on the builder, I need to start pricing out the 401 and 425 nailhead to see if it'll be within budget.
Anyone know the track width of a '55 Buick Super 56R code? I believe the wheelbase is 127" just curious on the track width.