I very much doubt its the gearbox at all tbh as Steve said the gearboxes with standard power are pretty bulletproof.
Its most probably the clutch dragging, and the synchros are stopping you engaging the gear because the propshaft is still spinning. Does the car go into reverse easily or do you get grinding? Grinding = dragging clutch.
On a 340 its even easier to tell. Get someone to look under the car whilst you operate the clutch. Lift the clutch in neutral and prop spins....dip the clutch and the prop should stop completely within about a second
If its not the clutch, then the second common possibility is the adjustment on the reverse gate....the bar that stops you getting reverse....you might need a little more clearance to get across as the linkages have worn. Adjustment is under the leather gaiter inside the car, under the gearstick.
The final possibility, is a wear in the gear selector inside the gearbox, but that's a very outside possibility.
