what tranny fluid do i use. HELP!

Trans fluid differs mainly in shift quality and resistance to temperature degradation. Type F is fine in a Mopar 3spd AT. The newer stuff will also work, and is insurance against heat breaking down the fluid. Mixing the types isn't necessarily a great idea, but will get you home. The reason you don't want to mix them goes more to knowing what the fluid is made up of in that trans. In other words, what percentage of each fluid so that the quality of the fluid is known.