To the OP: what amperage do you have now, and why do you think you need more? Keep in mind that the wiring in the truck is sized for the stock alternator, so upgrading the alternator also requires upgrading at least a couple of the wires.
As EOD Guy notes, there are may possible upgrades. You can get stock 12SI alterantors up to 94 amps and stock CS130 alternators up to 105 amps. Depending on which style you currently have (and 1986 could have had either), a higher-output version of the same style drops right in.