I have conflicting timing info in my chilton manual and under the hood on my truck. '75 k5 w/ 350 4 barrel and HEI. I just fixed (well mostly) a rough running problem. test drive elicited some back fires through the carb, especially at higher (that means above 2500 in my 150k mileage beast) rpm's. i would think that since the vacuum advance (hopefully anyway) is advancing as rpm's increase that this means my base timing is too far advanced. What timing are you guys running?