There are two different size starter motors for all Chevy engines. They will both bolt up to a SBC block, but only one will fit correctly. There are two starter motors because there are two different size starter rings to fit on two different fly wheels (or flex plate). There is a 10-1/2 inch clutch fly wheel used on six cylinders and base V8 engines; and an 11 inch clutch version used on all engines over 300 horsepower. The 11 inch clutch has 168 teeth around the starter ring and the 10-1/2 inch clutch uses a smaller 153 tooth starter ring.
Theoretically you can not interchange them, as the smaller starter ring is too small to engage a 168 tooth starter solenoid. It might get close enough to make contact but it would probably rip the tips off the teeth as they are not making good contact. The other side is if you used a 153 tooth stater solenoid on a 168 tooth starter ring it would run into it. Concievably this is what happened to your starter. Since it hit and engauged it probably bent the flex plate to the rear and is not disengaging because it is being physically held by the bent metal.
You can tell the two apart usually by the way it is bolted to the block. The smaller 153 has the holes that attach the starter motor to the block in a straight line side by side. The larger 168 tooth starter has the bolts offset at an agle so that the attaching bolts sit on a diagonal. I say usually because it is a Chevrolet and and as such parts are interchangeable even on a starter motor, so you could put a 168 tooth solenoid onto a 158 tooth stater motor if you wanted to, it just normlly isn't done.
Here are pictures showing the difference between the two starter bolt holes:
The black starter is the standard 153 tooth base engine starter while the orange one uses the 168 tooth solenoid.
Another and more probable answer is that the starter motor needs to be shimmed away from the block by way of thin brass shims (brass because it has to conduct a lot of current). You should be able to put a small paper clip wire between the teeth of the starter ring and the solenoid gear drive.