LD4B performer, RPM, and even m1 single plane none of em made a hoot of difference, maybe 1 MPG from best to worst and I doubt the SP2P would be any better without a corresponding drop in RPM. I think the important action is in the heads where it gets mixed the most and burned. I am a firm believer that as the TQ range of the manifold (and entire combo) goes down the overall combo would improve by matching the TQ curve by reducing RPM of the engine at cruise speeds either by gearing with OD, LU or higher rear gear ratio, the reason I came up with that theory is because of the zero deck tight quench 318 magnum I ran in a 98 ram 1500 ex cab long bed, tight quench and high compression gave me a little bit improved MPG and TQ but when I switched from a 245-75-16 tire to a much taller 275-65-20 tire it really got good MPG and loafed along at 80 with very low RPM (I think it barely did 2000 RPM at 80MPH), doing 70 it would easily do 20 mpg, that's what makes me think I could do better in a much smaller dakota by running a really high rear gear to keep the RPM down along with building the engine to make more TQ down real low.

Zero decking my current block, running .028 head gaskets with a 2.94 gear almost don't even feel like an experiment to me at this point, I would almost bet on 22 MPG in this little truck. That's why I am really want to push the compression and gearing in this thing.

If anyone knows where I can get some aftermarket heads (not chinese) for a magnum that don't cost an arm and a leg please let me know.