heavy engine braking & cheetah VB
67Dart273 - there's no forward movement in 2nd/3rd from a dead stop. .
THERE IS something seriously wrong.
The last thing I would do, just to make absolutely positively sure is to unhook the shifter from the trans and MANUALLY put it into 3rd On a reverse pattern this means the lever will be pulled all the way to the rear. Just count the clicks---start in Park, (all the way forward) and click 5 times--R-N-1-2-3, all the way back
Temporarily bypass your neutral safety switch by unhooking the push-on connector at the start relay, and clip lead it to ground. BE VERY careful as you will be STARTING THE CAR IN GEAR, and see if it will move
Use a helper if possible, use your parking brake and block the wheels until you are ready to "try" it.
This will tell you for sure that it's not a shifter/ adjustment/ cable problem.
The B&M's use a special shift lever, replacing the factory one. Did you use the right shift lever from the B&M kit?