The D5 was quite a different breed from previous 426 Hemi heads. The valve angle was much shallower than the D1/D4/D6 heads, which also made the combustion chamber much shallower. The intake and exhaust ports were round (and smaller than the standard Hemi), and had a much straighter shot at the valve. This is what changed the angle at which the intake manifold surface mated with the head (the manifold in the picture is for standard Hemi heads). Also, the intake and exhaust ports were much higher than the standard Hemi, and this is part of what made the D5 Colt front chassis a little different. Notice how low the front cage runners are going out to the front of the frame rails.
Another interesting change was the way the rocker arms were situated on the head, which moved the pushrods and changed the pushrod cutouts in the block to a different location.
Why the D5 heads? Henry Weslake designed them, and he had quite a reputation as a world-class racing head designer. He felt that the intake port was way too big on the Hemi for its intended rpm range, and thus the D5 had a smaller and round intake. And as for the exhaust ports, Weslake raised them alot and shortened the port length, getting rid of that more-than-90 degree bend in the standard Hemi exhaust. Chrysler thought they had a world-beater when those heads arrived, but disappointment soon followed.
Were they successful? Well, yes and no. The factory teams tried them on 426's (not successful), 396's (a little success), and 366's (better, but the 366 had other issues). The ports were just too small, and most or all of the factory teams went back to the D4 aluminum heads. Chrysler went back to the drawing board and came up with the D6 head design years later, the best of the factory heads in my opinion (leaky and porous castings aside).
The D5 heads were most successful at the beginning of the 500" PS motor days, as racers were willing to put alot of work into the heads to get HUGE ports out of them. While the D5 had small ports, there was alot of material there to play with, and some racers even welded in new material to make huge round intake ports for the 500" monsters.
