First, let's figure out what type of transmission you have.
Get under the truck and look at the transmission pan. If it is square, but one corner is cut off, it is a TH350. If it is just an odd shape, it is a TH400. If it is completely square (no odd shapes) it is a 700R4. You can also look at where the modulator is. If it is located at the back near the tail, it is a TH350. If it is more on the side by the dip-stick, it is a TH400. The 700R4 doesn't have one.
Being an '86 with a 305, more than likely it has a TH350.