Please Help us, my dad and i just purchased a show quality 1955 chevy and are having a problem with out tranny . . .it WILL shift into 1st and 2nd right away, but WON'T shift into 3rd . . . the vacuum line is connected, it has a saturday night special torque converter, and most likely a shift kit . . .what can we check? Should we put in a tranny that is lying around our garage that works, or could it be a minute problem that we are overlooking?

any help will be greatly appreciated
thanx