Why are you discing the corn stalks before moldboard plowing them? And you're even discing them twice? Seems like a waste of time and fuel to disc them. Kind of defeats the purpose of moldboard plowing them. And the plow may even do a better job if the stalks are left untouched.
IMO, sounds like the 5-18 is a little too much plow for that Massey. You should be pulling that plow 8-10" deep, and be able to move along at a decent clip, probably at least 4.5 mph or so.