Re: 351M Starter Gear Breaking
Either the supplier lacks integrity (or quality), your mount has shifted, or they're selling you a starter for a different application.
Can't give an evaluation of the supplier, you'll have to do some investigating.
A shifted mount would mean either you hit something really hard (you'd have known this) or the tranny and engine have momentarily parted ways and were't properly aligned on re-assembly.
Last: and this is very possible due to the age unless you've had it a real long time. Your engine/transmission was not meant to go with that car. Ford has been notorious for making minor changes from year to year on parts and they look the same until you actually hold them next to each other. If part of the combination was meant to be in a 76 pickup truck, for instance, the starter for that combination may have had a slightly longer reach than the 78 T-Bird combination which would mean your gear does not fully engage. The only way to confirm this would be to trace your castings.
Or you just may have a crappy vendor.
Chewing on asphalt and drinking dinosaur wine