If what you guys term as "dropping out of third" is having your tranny go into neutral by itself. Then i believe you have worn out 2nd/3rd gear sychronizer. Sometimes the brass insert in these drums just gets too much wear and wont keep the gear pulled into the drum. Or, thru speed shifting, sometimes the brass insert (which is just staked in) is pulled out of the drum. In the old days some would silversolder/braze them in (careful with the heat). If the i.d.s are worn too big, you can seat them a little closer to the gear (maybe grind a little off their length on the gear side)) and re-stake them a little less deep in the drum. I hope I've said what I actually mean! These brass inserts are a short cone shape.
Can synchro drums still be found? I think the '55 thru '64 synchros have different low/reverse splines but maybe they can be swapped into the earlier '54 and back cases when used as a complete gear/drum unit. The gears will swap-just use the complete sets together. There were the 2.94 low six cylinder sets; the 2.54 low V-8 set and the 2.20 low Corvette V-8 set. clutch gear, cluster, and second gear would make a set. In the 60's you could get a set from the chevy house for about $75. I knew a chevy parts man who put the corvette set into a 'XX thru '47 pickup box and with a little tail shaft machine work used it with his torque tube rear end. I'd cry if I ever broke that box!
In high school, I had a Hurst in my '54 chevy and would at times pull the thin shifting ring off the end of the drum. Between me and the 261, we could go thru some trannys, I was tough on the synchros, and the 261 was tough on teeth and shafts!
