The 1940 transmission will not bolt up to the the later style bellhousing. The transmission mounting pattern changed in 1948 to the "Modern Muncie Pattern".
The bellhousings will interchange between the chevy 216, 235, 261, and gmc 228 to 302 engines. That will give you a few more engine choices with little to no fabrication skills required.
Some of the suppliers sell an adapter plate that will bolt the early chevy/gmc 6 cylinder motors to an automatic transmission like the th350 and 700R4. The rear of the 230 motor would look exactly like the rear of the adapter plate. It might be possible to turn the TH350 to stovebolt adapter plate around to bolt up the 1940 chevy bellhousing to the 230 motor. This approach would keep your clutch housing and linkage in the exact same spot so the 1940 driveline would require zero modification. I havent't heard of anybody using this meathod to bolt up a later 6 in an earlier truck. It may or may not be possible to do. I haven't seen the plate up close. I can't think of any reason it coudn't work with little to no modification.
For what its worth, I have a 1940 gmc project that will be running 1957 gmc 302 in place of the original gmc 228. I am familiar with the clutch linkage/ brake pedal mount on the early bellhousings. My 302 engine and bellhousing will bolt right up to the truck with zero fabrication involved, however I will be running a more modern transmission and open driveline behind the motor. I will also be converting to disc brakes, so I will need to replace the Master cylinder and locate it to the frame.
