The engine is stalling when put in gears because the distributor timing curve is all wrong for the cam.
The timing is retarding when you put it in gear. ( wrong curve, light tension advance springs)
Recurve with a 10deg mech advance curve. 24-26deg at idle 34 to 36deg at max mech advance.
limit the advance curve travel to 10deg.
Then 10-12deg max of vacuum advance at hiway cruise. not all in at idle.
Now the timing will not drop when in gear (24 to 26deg BTDC minimum at idle, in gear)
4.5" power valve. The comp 280H cam does not work well with a stock torque converter.
get a 3500 stall for max performance ...3000 stall mimimum for perf and correct idling, in gear.