Sarting in 82 the 5.0 HO had the marine camshaft that has the same firing order as the 351. that was done to keep 7 and 8 from firing one right after another. the std 302 would fire, bang. bang. on the back throw of the crank, stressing it. if your cam is 180 out only 4 cylinders will be right if you are using the wrong firing order for your cam also only 4 clyinders will be right