An SBC engine in an early Ford V8 is about as simple as can be, the only problem is the fuel pump not having good clearance, an electric pump solves that problem.
I would suggest a GM 2004R trans in lieu of the 350/400. Of course you will have to alter the rear suspension, discarding the torque tube and early Ford rear end, I would suggest a first generation Camario/Nova 10 bolt, 34-1/4" backing plate to backing plate with a ratio in the 326.1 range. Stay away from the BPO rear ends, they were not good rear ends, you should also avoid any rear end newer that about '80, they are two wide for the early Ford chassis.
If you want to be a purest, keeping your '40 stock you can have a 59AB engine rebuilt and modernized, for about 8k, if you are lucky. then spend $5k for a Columbia that has been rebuilt and bullet proofed, all to please other people and drive the car about 2k a year to an occasional car show.