Re: Noob seeking help for solid build.
I would go in frame to keep the engine lower. I'm guessing you don't want to engage in a full blown project that requires modifying the frame. For a vintage type of frame the older Schwinn cantilever bikes have a lot of room in the engine compartment and look good. A number of builds here have been done with that frame and that engine/transmission combination. Look up Scotto's black and blue build I think it is for a newer type of frame which is aluminum. It also has lots of room for the engine/tranny. Depends on what you like. I have two Qmatics. One is running with a Greyhound engine on a 53 Schwinn Hornet... there's a recent thread on it you can look up to see how it sits in the cantilever frame. The other bike is a 50 Schwinn Panther with the Hua Sheng honda clone in it and barely fits in that straight bar frame. The cantilever frame has more room and is an easier build. Also, the HS engine is smoother, starts better and is much narrower than the Greyhound, so is more comfortable to ride. I have been away for the winter and used the Greyhound for my winter ride. Just yesterday I took the Panther out for a shakedown ride... started on the first pull after sitting since fall and runs so nicely. I always feel like I'm going to get where I'm going and home again when I ride that bike. That transmission is bulletproof. No, I'm not a dealer, but I'm thinking about it. Good luck with whatever you choose.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...