I have a 1975 Chevy Blazer with a either a 350 or 400 sbc motor. The vehicle originally had the 400 but the valve covers are from a GM Goodwrench replacement 350- engine. Is there an easy way to verify which motor I have? The engine is equipped with a Holley 4 barrel, single feed, manual secondaries, and a lowrise dual plane aluminum intake. The vehicle has never ran particularly well. Mileage is unknown. I recently changed out the spark plugs and now it runs even worse. I gapped the plugs to .035 which I would have though was good for the HEI and accel supercap I'm using. The plugs that came out seem to have been gapped smaller than this. They were all fouled, especially the four towards the back of the block. I think I may need a valve job on the heads. One tailpipe smokes pretty good all the time. The other side is clear of any smoke except a puff at startup. I am considering pulling the heads and taking them to the machine shop to have a 3 angle valve job performed, or I may go with a set of heads off a GM crate engine (I can buy GM crate engine head take offs for $300 from a machine shop here in town). Does anyone have any tests I could perform to narrow down the problem? I'd hate to invest in new heads and then discover the rings were at fault or it was just the carb or ignition. The bottom end of the engine is noiseless and oil pressure is steady at 30-40 pounds. If I replace the heads, I will also want to replace the cam and lifters at the same time. What cam brands and grinds have you guys had success with? I am looking for some power gains, but the truck is stock except for a 4" lift and 33" tires. I'm taking a long trip in this in July so I really want it to be right.