Starters nose cones break for: misalignment, not being secured or braced ,wrong type sholdered bolts (GM) , high compression (especially with timing advanced too far,causing "kick back") ,mis routed plug wires etc also "kick back" . Gm starters particularly rebuilts (due to case re -machining) must be checked for starter pinion tooth to ring gear tooth engagment . The little tool they give you is a "guage" designed to be inserted between the pinion tooth and the "crotch" of a pair of ring gear teeth . Trial and error ,shim, shim , remove shim etc etc the "listen" is a pain in the a$$ ,unscientific and can actually cause problems in the process. A loud "clang" on cranking typically signals need for shims because the starter pinion is coming out and "striking" the edge of the ring gear (not to mention bilge sparks !!) , also starters "sticking" engaged need shims or have low battery voltage , high pitched rotational grinding and growling could be due to too much shim (or not enough) or ... the old high pitched GM squeal/grind (pinion is missing engaging the teeth altogether). On Blower motors and very High compression we sometimes crank the motor ,then "throw" the power to it with a seperate switch secondarily . Enough for now heres a pic Hope starter 101 hasn't overwhelmed you. By the way to properly check the depth you need to pull the motor ,then use a pick to physically pull the starter pinion forward to check the engagement ,see pic (shim as neccesary) Tom

Thanks for all the responses from everyone.
I put the new one on, and ran into more problems. My buddy had rebuilt my Holley 3310 750 carb. He obviously messed up somewhere because the vent tubes or for marine applications "J" tubes were flowing gas. So I grabbed my buddies old Holley 750 and put that on instead of messing with mine. I got upgraded to mechanical secondaries in the process but lost the electric choke that I don't really need.
So we started it a few times and tried to mess with the water valve for the headers. So the starter did not make any weird sounds while cranking and was working fine. Then on maybe the 7th time starting it. My buddy saw the whole starter jolt. Then the grounding sound started. I thought for sure it was another cracked cone. It would not engage properly after that. We took the starter out and to my surprise it wasn't cracked.
So I think I will find that rear mount. Not sure where to start. The dealer?
I don't have a good way to remove the engine to check for clearance, so I am thinking about drilling a hole in the bellhousing cover to run the little rod/gauge to check for clearance. Any suggestions? I need to have the boat dialed in by tomorrow night to leave at 5 am Sat morning.
I am also having the suspicion that my flywheel mis be off. So after I remove the starter, I will check for that, too.
After the whole starter nightmare, I still need to lay one final piece of fiberglass on the bottom and do some sanding.