...I was once sold the Wrong starter motor for My 351 Cleveland. It tore itself apart. It turned out it was for a 351 Cleaveland with a AUTOMATIC Transmission (Flex-Plate). I had NO Clue! I was in Las Vegas when the starter burned out. 'It' would Start and Got Me Home...but disintegrated the Nose Piece! LOOK at It VERY carefully and Compare with a known Correct Starter! It's all about the Bendix/Gear Position and Length of Throw.
Because the Position of the (Automatic Trans) Flex-Plate Ring-Gear is CLOSER to the Starter Drive Gear...than it is on the 'Correct' FLYWHEEL, the Gear and Bendix, Never did Completely Disengage!!! Sounds like Your Problem. I did Not discover the discrepancy until I was back Home and replaced the starter!
Again...Good-Luck
MJ