Try a known good starter on loan from a fellow Model A guy and see if that solves the problem of the engine turning over slowly. If it does, you are back to a problem with your starter. Be sure to clean the metal where the starter meets the flywheel housing to get rid of paint build up, which can interfere with a good ground.
One other commonly overlooked cause of an engine turning over slowly is starter mounting bolts that are too long. The upper one is not a problem because it is above the flywheel. But the other two bolts if too long - even by a little - can rub against the flywheel and slow down the engine's revolutions. The bolts should only be 1" long with a lock washer. Check the bolt lengths and either replace with the correct length or add an extra lock washer to move the bolts away from the flywheel. This has fooled many an experienced Model A owner who took for granted that the starter bolts were the correct length. I also recall a story about a guy who answered an ad in the local paper about a Model A that was for sale. The owner had rebuilt the engine, but became disgusted because the engine wouldn't start. He gave up and offered the car for sale. The man answering the ad looked the car over, bought it for a very favorable price, asked for a couple shorter 3/8" bolts, installed them through the starter base and the engine instantly started. The seller was furious, but what could he do? The deal was done and the new owner drove away with the Model A.
Marshall