The rear slider is what puts the trans in 1st or 2nd gear, have you called Brewers for help yet? If not do so ASAP up
Do you know if your trans has the early side cover with the early style shift cams with the brass shift forks or the 1971 and later scissor type side cover with the steel forks?
If you push the rear slider back it is in 1st gear, if you push it towards the front of the tranny it is in 2nd gear scope
The slider should slide over both the brass synchro ring teeth and the tiny little sharp teeth on the gear to engage 2nd gear properly scope
Let us know what you see and find out wrench up

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)