Thread: Big Block Dilemma

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well the block should be a mark 5 with a fuel pump boss with that number but ?? i look the number up and said that number did have a fuel pump boss ? they could be wrong .all the older mark 5 did not come with a roller cams or fuel pump boss s from gm .some of the mark 6 did .i am not sure if your block has the towers casted in the valley for the roller hold down plate for the GM( spider) if so then its a mark 6 blocks other then this there about the same block 5/6all the 454 425 i have seen did come with rec port iron heads .i do not think the heads are right. GM did not use there iron oval ports sounds like someone may have put them on. if not rect port .if so older mark 4 heads they will not seal up on the new deck and will leak .now if there GM 502 alum O port 12363390heads i think they did rate that at 425 hp. get the head numbers take a valve cover off if needed to get casting numbers