SM-420 slips out of 2nd gear

[JerryCPP1]

The transmission is in a '57 5700 LCF. I just "did" the truck from the frame up, including disassembling, re-sealing, and assembling the transmission. Everything looked fine - the truck only had 48,000 original miles. Now that I got it running, all is fine except 2nd gear does not seem to go all the way in, and jumps out under any power or deceleration. I took the top off, and everything is still like new. The shift forks measure 4 1/8 outside, the same as the grooves in the mating gears. The shift tower works perfectly. I am at a loss. Any suggestions? Thank you in advance. Jerry.