Car dies in gear...

Your engine is suffering from one of two things; A) insufficient torque to drive the TC, or B) a too-tight or faulty TC.
Insufficient torque A), could be 1) lazy timing, 2) lazy compression, as in crossed wires, 3)Too-big fuel droplets,as in;restricted emulsion-tubes,restricted air bleeds, too-big IABs, or too-difficult a fuel pull-over, 4) a too-big plenum causing low airspeed,5) a too big carb causing too much loss of airspeed when going into gear, 6) slow gas; as in hi-test or stale, 7) some combination of the above.
Try this; get the rear wheels off the ground. Start her up and put her in gear. Let her idle. Start applying the brakes while watching the tach. If it stays running, what is the rpm difference from freewheeling to wheels stopped? If it stalls when the wheels stop turning,there's a good chance the TC is bad.