A 53 1/2 ton with column shift should have a 3 speed, not a 4 speed SM420. Are you sure you have a SM420?

In any event, both trannys are synchromesh, except for 1st and reverse, so I would think your synchro is going bad.

Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix