svcattle - 1st off, Welcome to Stovebolt!

I did a little searching in
GMPartswiki and found there are 3 different part numbered starters (all identified as heavy duty) listed for 1972 5000-5500 Series GMC trucks with the 350 engines. Need to know which transmission your truck has to refine the info and try and give you a number and possible pointers to a source for your starter.
Please be specific on the transmission identity in your reply and I can try and help further.
PS - Kevin beat me to the reply post button - LOL. I would note that the pictures of the starters I found searching NAPA don't match the starter mount bolt pattern in your pictures. May or may not be a deciding factor as NAPA often uses stock photos which aren't exact matches to the products.