I have rebuilt a 1960 DBD34GS.
When you insert the pipe it should hit a hard stop against the internal step in the head. If you feel the hard stop, is the mounting bracket directly in line with the engine mount stud, or do you have to slightly pull the pipe back out of the head in order to insert the stud? If the latter, or if you don't feel a hard stop (because it binds before reaching that point), that may identify the source of the air leak, i.e. the mounting bracket is either bent or, in the case of an aftermarket pipe, incorrectly located.
Any air that makes it as far as the internal step has to then make two 90° turns as well as squeeze through the remaining gap between the pipe and step, which is pretty hard to do if the pipe is pressed against the step. Note, if clearance stops the pipe before hitting the step it doesn't mean the fit is tight around the entire circumference, only that a minimum of two points are tight, with the rest of the circumference free to flow air.
When you are ready to fit it, run the bike without a pipe for a couple of minutes to expand the exhaust port then slip the pipe in Quickly
Having cold air sucked back over a hot valve stem isn't a good idea, and this would be especially the case with the large valve overlap of a Gold Star. Also, if the extra ~0.005" of a hot head is needed to install the pipe, the fit is too tight anyway and only would cause future headaches whenever the pipe needs to be removed. It doesn't have to be a hermetic seal to eliminate the backfiring problem.