Welcome!
So far I think you may be better off finding an original replacement frame for what you plan to do.
If you do end up swapping frames take a bunch of measurements before you completely decide it is the right thing to do. As Steve points out, the track width and fitting tires under the fenders will be the big challenge with a newer frame.
Personally I think the original style frame is the best choice. The cab and front sheet metal already fits and looks good on it. In your case the engine you have in mind will be a good fit. And with that engine and your planned use the stock axles, gears, and brakes should suit your needs.
Then if you want better axles, brakes, and gear choices you can still use the old frame, and it may be simpler to swap axles like I have done, than it is to swap an entire frame and drivetrain. But either one is still a lot of work.
Grigg
