2 differant questions

Alright both these questions apply to my 92 k1500 5 litre. First off I have been having problems constantly with my ac/heater and my radio/clock I've traced the problem back to the little circuit box(not sure if that is the actual name for it but its a metal little box that all the wires run to and the antenna plugs into also) both units run wires back to under the dash to this box my question now is....I have a parts pickup with a box in it but the only thing differance is in my parts pickup the AC/heater control unit doesn't have a re circuate button so will that box not work?

And then the other question is about tires I got a set of 285 75s for free and through em on my pickup and now it can barely get up to 80 and when its above 65 mph and Cruz is on it constantly downshifts the transmission even on flat ground to keep the speed up so I don't think those tires are going to work unless you guts have any ideas but my question is would 265 75 be just about as bad or no?

Come on someones gotta know. (and i know different is spelled wrong im not illiterate but i typed it from my phone and couldnt see due to the small size and i cant edit the title.)