I would not build that bike with that engine. It does not make sense to use it in a featherbed frame. Tritons were built because we could not buy Manx engines. Some Manx Nortons were available without motors, because the car guys used to buy a Manx to get the motors for 500cc car racing. A 1963 Bonneville was a very good motorcycle. The bike in the photo looks good, but the gap behind the gearbox - ? - 'Because I can' is never a good reason to do anything.
With that frame, I would use a Norton twin motor with a separate gearbox. For any sort of historic value, a Triton needs the wideline frame and a non-unit motor.
Whenever I build a motorcycle, I try to make it correct for its period. If I had that motor, I would look for an appropriate frame. The bike in the photo would make a good 650SS.
I am sorry if I have rained on your parade, but I have built about 5 motorcycles out of parts. I always think a long way ahead before I begin. The main stumbling block is usually the gearbox.