I have a 41 Buick and was thinking of the frame swap idea. The cheapest option I found was a cadillac swap. It had the closest dimensions and you can pick up the frame 200-300. I was talked into a camaro subframe then the person doing the work flaked on me and I ended up calling Streetrod engineering in Arizona, Buick is not your run of the mill car, but they took the dimensions from my drawings and hooked me up with a completely new front end.
I even bought one of there universal rear kits and they made additional mounts for me at my request. But to say the least I am actually going to use the original rear setup, slightly modified for a 10" rear something I was not optioning before. My $400+ universal kit is now sitting wasted on the floor, but the original, as you know resembles a 4 link suspension on steroids. and it is a lot cheaper. $0 vs 400 MMmmm.... Ok the front end cost me a bit, but it was worth it, all the holes match and I do not have to worry about fabricating anything extra since it was built for the car.
Just a note on the rear end, I plan on removing the stock springs and putting in late 70s monte carlo springs which will give me ~3in drop overall.