My truck has a 351W with a ZF S5-42 4x4 trans. I have replaced the timing chain, clutch, starter, and the alternator Has been replaced twice. I am wanting to know what I can do to max out the general performance of my truck. I have been having a few problems with the engine at idle speed. When it starts cold, it revs up to the 1000-1250 mark then once warm knocks down to about 750-850 where it should be, but then after driving it for a while, when I pull up to a stop sign the idle will stick at about 1500, but then if I tap the gas fast it will rev up and knock back down to normal idle around 750. Could this be a problem internally with the throttle body's butterfly valves? I have been told that I should swap the Speed Density sensor for a Mass Air Flow sensor. Could this fix the problem with the idle?

If your truck was originally a 351/5 speed truck switching from speed density to mass air by itself will not fix your problem. Now, if the truck has been converted from auto to stick, the computer may not be happy. A computer swap from a same year/equipment truck will likely fix your problem.

If your truck is original, I would make sure the throttle cable is not sticking, then I would clean the throttle body and the idle air control sensor, check your TPS voltage, and pull codes.

But basically, no, switching from speed density to mass air will not fix your problem.