slow cranking starter

Stock starter should be more than adequate, you say the battery has been checked but you may try putting another battery that you know his new and hot. You also May check your timing too much initial timing and it will definitely strain your starter I also remember that some ome starter spin faster than others but I can't recall the years or specifications someone else may chime in, if I recall it was after 73 and it was to help start engines with the crappy gas