There were no 300 ci gm sixes. As Don suggested it may be a 302 GMC. If it is you are lucky! Easiest way to confirm this is to check the number stamped into the flat area that the distributer bolts to. To the rear of the distributer there is a number. If it is a GMC (truck engine) the first three digits are the displacement of the engine, 302, 270, 248, 236, 228, etc. If it's not a GMC you must go to the casting numbers. There are tec article that will take you through a 12v conversion. You can keep your 6v starter. Tom
