I agree with Jerry as far as the diesel being the best way to go.

If you want to stay with a gas engine, personally I’d find a 500 CI Cadillac, do a stock rebuild and swap that in. Even the low compression regular gas engines from the 70s made 500 Ft Lbs of torque at 2200-2500 RPM and were extremely reliable. You would need an adaptor for the TH400 to fit the caddy engine but they are readily available.

Finding/buying a 500 Caddy engine and doing a quality rebuild would be far cheaper than a custom built BBC 496/511.

If you decide you want to stick with a BBC, Pat is definitely the one to talk to.