I recently replaced all bushing from the firewall back, and patiently adjusted the pinch bolt at the shifter. All gears are there now. Small issue with fourth - Once I let go of the lever it 'jumps' to the left. Fifth engages ok. There is a lot of play in the ball cup. Normal or a result of nearly 50 years of ignoring the lube?

Do you mean jumping out of 4th ?,I presume you have a tailshaft box ? have personally had no experience with them but I can say you need to replace EVERY part ,any piece left out is the weak link in the system.