If the bendix is not returning all the way, it might be the bushings inside the bendix are damaged or coming loose and jamming it.
The fork that throws it into gear could be installed backward or damaged or worn.
The return spring could be too weak or soft.
You didn't say if you could manually retract the bendix all the way or if there was something preventing it.
There are two length return springs, the short one works better for hot starting problems, but the disadvantage is that there is less spring to pull the bendix back.
The fact that you had to put an external solenoid on the car tells me perhaps you have something binding the bendix.
While you are working on the starter, pull the end cap and check the brushes for wear. If they wear too far the starter will lose power and eventually the screws holding the brushes will hit the armiture shorting it out.
I use a Standard Motor Products #SS200 Solenoid. It is teflon lined and uses brass and copper fittings. It uses the short return spring.
Check my web page for First Gen Camaro suspension info:
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer