[RICK LAKE]

Large Arbor Phil if you are having starter problems and have a good battery and starter here's the problem with eating teeth on the flywheel
I had the same problem with a shelby block motor, 3 choices
I don't know which bellhousing you have but if it's a lake wood your mini starter doesn't have enough extendion for full engagement. Does your bell housing have a block protector plate? If yes You need to remove where the starter goes through the hole area and have the starter mount to the bell housing directly. I sabre sawed mine with a metal blade. The thing with this is checking the starter to make sure it is not hitting or rubbing on the block. I had to grind a little of the block to clear this too. Also and this is BIG, Make sure the bellhousing is centered on the block. Alot of problems happen from this not being done. This includes Stock bellhousing and Quicktime or any other ones. Mine with .038" out. After 3 hours of trimming and adjusting I Got it down to .003" max spec is about .005-.007" out of center.I have found that allen head bolt on the blind side of the starter work the best.
Once we know we have the starter mounted deeper for more full contact we need to look at the air gap between the teeth of starter and flywheel. You want about.035". The easy way is to just use the flywheel without the clutch and ingage just the soleniod of the motor. A large paper clip is the correct thickness. You should be able to stick this between the 2 teeth. You don't have to be perfect but in the ball park. The wider the gap the more stress on the teeth of the flywheel. This job is a pain in the A$$. As for the flywheel, you can have just the teeth replace. If the flywheel has been worked hard with cracks and burnt marks, replace it.
I starter with a Tilton starter and worn out 2 drives before finding out what was causing this failure. It's a pain in the butt to jump start the car.
Other side note, if you are running a Lakewood bell housing and a GM input shaft trans, measure the depth of the input to the back of the block. In some cases you have to remove .280" off where the tip goes into the bushing or bearing in the back of the crankshaft. IF you don't you WILL wear out the thrust bearing permaturily. The is a small side note in the instructions from Lake wood on this for FE motors. Not sure about other Ford motors.
The other fix is to go old style starter and hope you have a strong one and heavy cables to handle the 150-250 amp draw on starting the motor. The OEM starters can be a pain in the butt when they are heat soaked for a long drive and will barely crank the motor over. This is the main reason for the mini gear reduction starters and they draw only 70-120 amps.
The last choice is to have the starter mounting surface machined about .250" and this will give you the added extension on the drive to give you almost a full contact area and stop breaking the teeth on the flywheel. Sammy here on CC has done this and it worked good. The only thing a don't like is the fact of a thin mounting surface with 3 small bolts to hold and control the torque twist when starting the motor. If you have a problem down the road with this starter, need to have it machined again to fit and work correctly. This is the best info I have on this problem. Good luck Rick L.