That's odd. I don't know the specifics of the 4.2 Brison weights but I know that when he changed from the blue case 3.2 to the polished case 3.2 the only things that changed was the color. Same with the 6.4 twin with the later exception of the prop shaft diameter. Weights between those four engines remained the same. Early mufflers for the 6.4 were heavier than current versions but those are bolt on items.

If the 4.2 being looked at was the old A&M case it would be understandable.