Hi, I think I can safely take credit for starting the whole headlight conversion trend on this board...
As you correctly surmised, an 89-91 truck will use the H7 bulbs. You don't need specific part number recommendations - A search on google or similar will turn up dozens of H7 choices in all different colors (blue, yellow for fog, super-white), wattage, and price. You can use whatever you want really.
As far as wattage, the difference between high and low beam wattage is very small or sometimes there is no difference at all. What makes the high beams "high" is the design of the glass and reflector in the conversion adaptor. As stated above, there are many bulb choices 55-150 watts. Some of the cheaper bulbs on the market claim much greater output than they can really deliver, so don't be afraid to get up in the 80-100 range, as the true brightness will be somewhat less. Oh yeah, the H7's only have one filament per. They're not combo bulbs, which is why you need four. Hope this helps.