Just like engine survival oil in & out. All clearances must be checked.
Inside gear diameter & mainshaft outside diameter & end clearance are all critical to transmission survival. Oil has to circulate in & out.
Toploader 1st gear to rear thrust and to 1&2 synchronizer were always on the tight side of end clearance. Many need custom a ground thrust washer plus extra oil slots on synchronizer & thrust are a plus.
Minimum.010” end play is needed between gears / thrusts / synchronizer hubs.
Gears to mainshaft .003” minimum.
All parts need to be checked , the tolerances can add up to catastrophic failure.
1963 1/2 Galaxie 500 390 Z code 4 spd.