We have a 73 Mach 1 that had a 302 with a C4 originally from the factory. The 392 engine had a cam gear and distributor set of meshing gears that got into a fight. Both lost. He replaced the 302 2v with a 351W from a 1994 1994 F-150 truck. he uaed a set of heads from 1969 351W 4v engine (better breathing than the 2v heads, and 1969 is the only year the 351 4V was offered in a new car, although I have heard some 351W engines made it into 1970 cars once in a while when the 351C engines were not available. If that happened, and there seems to be some evidence to support that "rumor," then it is maybe possible a few of those were 351W 4v engines - maybe...
In any event he had the engine built for moderate street/strip performance, and kept the compression down to 9.5:1 even with aluminum alloy forged high performance pistons in order to be able to run on lower octane alcohol free gasoline out here (89-90 octane). It ended up pulling 360 HP on a dyno, definitely enough to have some spirited performance.
Anyway, back to your issue. It seems our trajectory on our Mustangs is similar re: starting with 302 engine, then moving to 351W engines. In our case the tranny was a C4 automatic. The other year I swapped in an AOD, and the engine-transmission separator plate I had purchased was stamped a little off where the starter motor is mounted. We ended up having to beat the plate's arc where the starter motor passes through a bit to let it get bolted into the bell housing with its two (2) bolts. As it turns out, that amount of room we gave it using a hammer to beat the opening a little larger was not quite enough to prevent a noticeable interference whirring of the starter motor when it was cranking the engine over. Once we had the Mach 1 back home I put the car up in the air, removed the two mounting bolts and used a rotary carbide cutting tip to remove all the excess steel plate material that was interfering with the start mounting at the proper angle with no outside interference. The cutting away of the excess separaator plate did the trick. More to the point re: your situation, it was definitely two bolts I had holding in the starter motor.
I dug through my various videos I took during the AOD transplant process, and found these two images of the original separator plate both before and after the flex plate was removed. It is clear we only had two mounting bolts for our starter. If your Mustang initially had a manual tranny it is possible you had three mounting bolts, but I do not recall that being the case in my prior under-the-car times from decades long gone by. Given the input you have received from other well qualified folks, if they mention a possibility of three mounting bolts I would pay heed. If it is not possible for you get a look in those tight places it would be worth getting a self-illuminating bore scope and running it up above the starter to see if there is a 3rd bolt.
I recall every once in while running into a starter motor that was really hard to remove even after the mounting bolts had been definitely removed. Nothing a heavy, hand help brass mallet could not handle, although I was very careful to not strike excessively hard or on cast metal parts. Just a few sharp raps as far from the starter gear end ("shock leverage") as possible is all it took. Once I got a little movement out of the starter motor it became easy to get it out the rest of the way.
Good luck. I hope you post your findings once you get the starter motor out.
