I have a 67 RS with a stock 350 pulled from an el camino. This engine is pretty stock but i noticed that when the car is idling and you floor it the engine stalls. This is more noticeble in reverse. It seems like the car either loses spark or fuel. This only happens when the car is sitting still, if it's moving and you floor it it works fine. The car has a mechanical fuel pump, accel coil and distributor. I have no idea where to start trouble shooting. ANy ideas?