changed transmissions700r from my 91 firebird,the tranny i put in has a metal gear on the rear shaft with a elect.speedo with no gear, same elect plug in,mine had plastic gears on tail shaft and on speedo. i plug it up the speedometer and it jumps to 120mph and dies on take off.i unplug it and cars runs ok.Q can i change the gears out?

 

yes. provided the van and firebird both have a v-8 or a 4.3.
the 2.5's will interchange with each other also but not with the v-8 or 4.3.