what kind of battery should my 1986 s-10 2.8 have

what slows down? I don't understand what your question actually is. It's normal for the voltage gauge to drop a little when running a lot of electrical stuff. The A/C will cause the engine to feel sluggish because it's a low power engine and the compressor takes away a significant portion of that power when it's being used.

you would think you need an alternator but you need a multi tester to see if it's charging normal and under load , voltage drops , body grounds ,the alternator needs a good ground ,,any number of things in the electrical circuit(s) and the like are possible with a vehicle that old , hopefully it's something simple .

if that battery is old like the truck and has the pop off caps then it should be replaced . Better than getting stuck , let us know what happens .