The first year (1976) tractors had four pinion differentials. Second production beginning in May of 77 were built with two pinion differentials at S/N 277 182. The 2-85 has steel reinforced ring land pistons where the 2-105 did not, thus is a better engine.
Be careful with the smoke screw. Engine now has more torque, so add duals and more weight. Now you have more HP and tires that are glued to the ground and something in the middle has to give, humm two pinion diff.
Never ceases to amaze me, buy a Yugo and add a couple things and it will go as fast as a Corvette?