About 1978 I had a F 1 that would slip out of second under a heavy load. Such as giving it too much gas climbing a steep grade.
Back then people said it was the gears that were worn and would cause the transmission to slip out of gear. Others said then bearings were to worn and that allowed to much play in the shafts when under load and the transmission would slip out of gear. Take your pick.
My dad I were able to buy all new gears and bearings and rebuilt the whole transmission. Lot easier then trying to remove the bronze bushings in the main cluster gear and find new bushings to be pressed when you could buy a brand new cluster gear.
So if the parts are available try to rebuild it.
Jeffrey
