i have almost the same bike. it's a 1956 goodyear hiway patrol special, made by columbia. our frame's are the exact same.
the motor's a real tight fit. i had to use a smaller front chainring and lower the chaingaurd to get some clearance.
once the motor's in there, it fits like a glove. didn't even need the chain tensioner.
as far as the brazed joints go, i was worried about that, too, but mine's held up to tons of abuse with no problems (yet.) these are pretty strong frames. if yours is in good shape, i wouldn't worry too much about it, just keep an eye on it.
since we got the same bike, lemme know if you have any probs putting it together, i probably had the same ones, and solved them already.