Hopefully this is good news for this guy. A clutch will not cause a trans to pop out of gear. When they removed the trans to do the clutch your linkage must have been altered/adjusted/tightened if it's not popping out now. Fingers crossedPersonal experience. Started slipping a bit in 3rd gear and occasionally would pop out of 3rd gear. Big exception was that I had close to 100k on a Stillen clutch unit at that time. As we all know, clutch life time generally has nothing to do with GT mileage. Same can be said for rear tires. Very few Little old Ladies from Pasadena in this group.
Syncros won't cause a trans to pop out of gear. They do nothing once it's in gear. They'll keep it from going in or allow a grind(2nd issue).
Popping out is caused by not going into gear completely. Linkage, blockage, forks on the easy side, to worn out energizer teeth on the slider and/or gear and excessive trans endplay.