I have seen the issue with insufficient tooth engagement on multiple occasions, including with Powermaster starters. Nowadays before I install an aftermarket starter I actuate it on the floor of the shop and measure how far the gear kicks out, then measure from the block plate to the back of the flywheel gear, and see if the measurements match. A lot of times you will only get 1/2 of the teeth engaging. The only solution I have found for an aftermarket starter like this is to mill the front face of the starter down, so that it sticks farther into the bellhousing. That seems to solve the problem.
On the Robb MC starter, I had an early one where the one way clutch gave up; sent it back to Robb and he fixed it and shipped it back at no charge. Its been working fine every since. This is the one with the nose support for the shaft; does your starter have this? For a big engine, I think that is pretty important. The Robb MC starter I have engages all the way across the flywheel teeth, as opposed to the Powermaster, which does not.
You might consider sending your starter back to Robb and see if he can repair it for you; I'll bet that he could. His starter cranks over my 13.4:1 577" SOHC with no problem. I doubt the problem you are seeing is an issue with the starter not having enough torque to last. Probably some other failure, maybe burned brushes or the one way clutch that went out on my first one.
I sure wish I could get away with using a stock starter like Rory, but they have never worked for me on any high compression FE. The stock starters just won't turn the engine over when it is hot. The heat from the headers in an early Mustang chassis, running right by the starter, certainly doesn't help. On the other hand, the Powermaster starters (after modifications) and the Robb MC starter work great.