Which automatic depends on you and your truck. If you still have the stock rear end, it should have either 3.90 or 4.10 gears, iirc. These are not good for highway cruising. With them, I would recommend a TH700R4, which is a 3 speed with a .7 overdrive. Buy one that has been upgraded, the early models were lacking on some of the internals, but will work great when built to later specs.
If your gear ratio is under 3.42, you will want to consider the TH350 or TH400. The 400 is a stronger tranny, but has a considerable power loss. The TH350 can take almost any small block you put in front of it, without the losses of the TH400. Both are 3 speed autos.
There are also the powerglides, but they don't have the durability of the other boxes without major improvements, and they are a 2 speed box.
I wouldn't recommend a hydramatic. They were a good tranny 40 years ago, but finding anyone who can work on them is getting tough, as well as the prices going sky-high.
