There is no Plymouth Magnum, that is the Dodge name for the high performance engine. Plymouth called them Super Commando. To the bes tof my knowledge all RoadRunner 383 engines were 335 horse power. cosgig is right about the heads, in '68 they went to open chamber heads from a closed chamber. Given everything I've read it looks to me in '68 the pistons were a little higher in the bore to make up for the bigger chamber and keep the compression up. Today, if you just by a replacement piston they have 1 part number for all, closed and open chamber heads so you would need to know that else on a '68 up you will kill your compression. All this applies to the 383, I don't know about the pistons in the 440. The RR cam was also used in the 440s, it was a little more cam than the stock 4 barrel cam. One big thing that helped the automatic RR cars also was the torque converter. Back in the day they called it a Hemi converter, it has a higher than stock stall, maybe 2200 rpm. My guess is this was the standard converter for an automatic Hemi. like I said, my guess. My car has a 383 so that is what I researched. If you want hard details check out Uncle Tony's Garage on UTube. He has forgotten more about big block MOPARs than I will ever know. Hope this helps.
