What ever you do do not cut the dash harness plug out. All 91 trucks are wiring harness specific. I learned the hard way other year trucks will plug right in like no big deal. But...in 91 they moved the wiring pins location in the plug. Ignition, fuel pump, steering column control circuits wipers,turn siginal, high low beans and those are just the ones I found with the truck I was working on at the time. Use the factory plug get your power from the fuse box your accessories side from the original plug so the lights will all work together the same . Use no less than a 4 gauge power feed wire with a fuse or circuit breaker to protect your wires from heat. Run your wires opposite side of the trucks wiring under the carpet to stop noise clutter out if the speakers. 10 inch sub will fit nicely behind the seat where the jack mounts power amps needfree air flow around them and look nice when mounted in the jump seat hole. And use the bottom seat plate to hide it all for display an led light inside the area comes on when you open the seat plate to show off the amp. I installed a bunch for the car club guys. When you park it hides to prevent theft and broken windows.
As far as the dash speakers are concerned, pretty much any speaker other than OE are going to require some persuasion to squeeze in there. I used Rockford Fosgate Punch speakers and had to do some trimming to the upper rear corner of the gauge cluster surrounding, not much, just enough to be a PITA.
You won't believe how much better the sound and power flowed from a good installation. Or how many butchered trucks wiring I've had to repair. If you don't start out on the right foot you'll trip and fall.
Adapters for harness yes, at the head unit. From the head unit use RCA outputs and make sure your head unit has three RCA outputs. The increase in power will work much better thru rca cables. The speaker wires are more visible during connections and keep flowing in the right direction. A four gauge power wire to the amp with a good circuit breaker to protect your investment. Mount the amp with at least a half inch of clearence under it for air flow reducing heat. A capaciter to prevent alternator issues will also save you money 1.5 mf will handle a thousand watts of power. I've set up two amps and run 1,500 watts in an S10 extended cab using the jump seat hole for mounting and LED lights to show the mounting at car competitions.