As for upgrades, a gear-drive starter is half the size & weight of an OEM-style direct drive starter, so its easier to handle while changing, which also happens if you ever need to R & R the ZF in the future. And being smaller it sits further away from the red-hot headers so it runs cooler. It also develops 30-50% more cranking power than OEMs. An OEM starter is marginal in a high compression 351-C when its hot. Most hot rod shops sell cheap starter heat shields if heat-soak is your problem. But as was said, most electrical problems in a DeTomaso traces to bad grounds. Some owners have heavy ground wires to both the engine AND the ZF, connected to a clean bare part of the frame.
Also, OEM starters used phenolic insulators for the contacts inside the assembly. Cheap rebuilds (or brand new far east 'bargains') use cheap poly insulators. 300 amps of starting current for very long (over 10 seconds or so of cranking) heats the internal contacts enough to melt poly so the contacts sag. The next time you try starting, no electricity gets to the motor windings. If you're lucky (I was!) the 40+ yr old phenolic insulators- which do not melt- in an old starter may fit your dead 'new' starter.