The 400T motor is a stressed member of the frame in your bike. CB750 and 350 motors are not. 350 wouldn't be terrible to mount, but you would probably have to make a set of lower frame rails going from the end of the current front tube to the rear of the frame. CB750 would be the same, but you have the issue of it being much wider than the 400T engine. Also, the backbone on the frame could give you issues with clearance for carbs if you're swapping another engine in.
If it were me, I'd look for a CM400 or CM450 motor to go in, they're basically the same with the exception of the 450 motors having an oil cooler on the front. Shouldn't be any trouble to get one in your frame though, they're mounted the same.
What parts do you need? I have a complete motor from a 78 CB400T.