Back then, the engine block of choice was the 340 6-barrel block. However, by the late 1970s, 6-barrel blocks were hard to come by and we were scrounging the local junk yards. It was the same thing for the Pettys. We were using standard 340 blocks for dyno development and saving the six-barrel blocks for the race cars.
We wanted to use the 6-barrel block was because we had added material to the bulkheads for strength, so we could use 4 bolt main caps. Otherwise, the only difference between the engines was carburetion, camshaft, cylinder heads and headers. The NASCAR engine could only use a flat tappet cam (we used a mushroom tappet) and a single 4 bbl carb. The NHRA drag engines could use roller cams and multiple carburetion intake manifolds depending on the class.