Could be mechanical, like a starter that isnt shimmed right(too tight) or an electrical issue like a weak ground to the ford solenoid,causing it to act like a shunt,when current draw is high (like when cranking) it stays in, after engine starts, current draw goes down (less power required-engine running) , when current draw drops, solenoid releases. A bad ground can exaggerate the "shunt" condition. I have had a poorly grounded ford solenoid cause a starter to stay engaged a few times in my career.
Also the ford solenoid could be mechanically sticking internally as well as the GM solenoid on the starter could be hanging in. Could be rusty or in need of a stiffer return spring. Just a few thoughts.
How is the ford solenoid wired in? Purple wire from key to ford solenoid-large terminal to S terminal???