Probably other stuff like the fuel system as well (fuel pump, fuel pressure regulator, larger gas tank.) And of course, when you go to the correct trans, you'll need a driveline and maybe different linkage. Front suspension? Rear end (gearing?)
Can you find a parts truck? You'll really need one.
Is your 4 cylinder dead? You might be ahead financially to dump the 4 cylinder truck and get what you want. Seems to me like you're looking at a whole lot of work time, down time, and expense to get a performance bump.