I am replacing a sick 228 with a 270 in my '53 GMC 1/2 ton. The truck has the hydramatic trans which I want to keep due to it's uniqueness. The 228 engine Zenith carb had the automatic choke which I understand were standard with the hydramatic. I swapped the 270 Zenith throttle body, venturi and jets to the 228 carburetor. The engine runs well at idle and higher rpm but when driving runs very rough but I can't be certain if the problem is fuel or timing. One mystery I haven't resolved is not getting a spark from the coil wire to ground when setting the points, only a very slight spark on the points when they open and close. I would expect a bad coil but the engine does run well as described above. So, a few questions I am seeking help/advice...were the carb modifications correct to use the 228 carb on the 270, is a bad coil the probable problem with setting the points and is the correct position for the timing pointer at 5 degrees BTC for the 270.