I am not a slant 6 expert but I would have thought by 74 the /6 would be using hydraulic lifters? Anyway, there are basically 2 types of cams (as it relates to this discussion), a roller cam and a flat tappet cam. within these 2 "categories" there is solid and hydraulic in both styles.
On a flat tappet cam (solid or hydraulic) the base of the lifter rides on the cam lobe directly. The lobe raises the lifter up and down as it rotates and the came imparts this movement to the pushrods which in turn push (hence the name) the rocker arms which "rock" and open the valves.
On a roller cam, the lifter has a wheel in the base which "rolls" on the came lobe and does all the same things as the flat tappet cam.
