Almost every machine shop has a 350 laying aroung that somebody left them without going ahead with some machine work. Can't be hard to source locally, too.
But I have an opinion that you didn't ask for. (I know, I know). Looking at your project progress, I'd say you're not in a hurry to get a motor in it. Plenty to do in the meantime.
For instance, summertime is a great time to clean and detail the frame and axles.