1966 Mustang - Engine Won't Start

Hi,

I own a 66 Mustang, I6 auto C-4.

I recently replaced the alternator, ignition switch, starter, and starter solenoid. Everytime I try and start the car, the starter motor stays stuck to the point where it sounds like the car is at full throttle without even depressing the accelerator.

I disconnected the throttle linkage and even bypassed the solenoid with a screwdriver, but the starter still stays stuck(the speed at which it's turning sounds like the engine at full throttle). I had a mechanic show me this trick and he was able to get it started a while back, however recently the car either cranks aimlessly or the starter catches, there's no in between.

Took the starter to get checked and it passed. I'm completely out of ideas and paranoid about turning the car on given the fact how many times the damn starter has stayed stuck.

Any help would be appreciated. At the end of my wits here. The car has been sitting for six months