since you want a bench seat and a/c your options are gonna be to use an off the shelf aftermarket heater a/c unit, like vintage air or similar, and lose the use of the glove box. or, use the universal under dash "freeze you knees off" system, which will be cramped for space if you are the "middle guy", or, try to fit a unit from a donor vehicle that mounts a bunch of the parts on the engine side of the firewall to free up cab space and keep things useable inside. you could look at some of the builds that joedoh does and see how he makes that work. if that is appealing to you then possibly score a heater/a/c unit from an s10, or another donor, and go to town cutting and fabbing to make it work. basically you would copy the cutouts from the donor where the unit attached to the firewall. you can always plumb the venting into the dash wherever you like, once the main unit is installed, and use aftermarket vent louvers available from suppliers like vintage. it will mean your truck will lose that stock dash and under hood look but you will have the bench seat, reliable heat and defrost and nobody will have frozen knees or sore legs from being cramped. since you are using a small block for power you should be able to find a compressor, pulley system and brackets to mount the compressor on the engine and any hydraulic hose supplier should be able to hook you up with custom hoses and fittings to fit your needs. I suggest to strip the complete package from a single donor so all the parts work together like they were made to. if you have the time and tooling available you could even cut away that part of the firewall from the donor so you have a pattern to work with as you may find there are humps and bumps on the donor firewall that correspond to irregular shaped areas on the actual heater box profile. find an S10 or donor vehicle with a small block or a 4.3 engine because the 4.3 will use the same basic profile as the sbc for mounting stuff to the engine. the 4.3 parts actually fit a bit closer to the engine, I think, and they may use some stamped steel bracketry instead of aluminum sometimes. be aware of water pump lengths and offsets and know your clearance to the rad on your set up before you go. maybe joedoh could answer some of that for you. that would likely get you a serpentine belt driven system if you are looking for a more modern a/c compressor that is made to pump R134A. if at a wrecker you could come up with a serpentine belt driven system from another stock chevy vehicle, other than S10, that should bolt up to your sbc and include an a/c unit. just be aware that some of those serpentine systems may have the water pump that is driven in an opposite direction than the older style belt system. your engine is apart anyway so swapping a water pump will be easy. these systems could put your a/c compressor and alternator up high on the pass side so connecting would look less cluttered under the hood. some systems run a smog pump that can be deleted. some have a more spread out configuration. some have the a/c unit on the driver's side. be sure to grab the a/c compressor for your proposed system to ensure it works properly with the refrigerant you will use. likely R134A because it is the more standard and readily available type. also grab the alternator, power steering pump and hoses, all the tensioners, the interior heater controls and even the wiring harness ,or at least the plugs with a pigtail of wire left, in case you need to convert to a newer style alternator, basically grab everything in the system that bolts to the front of the engine. accy's tensioners and idler pulleys, coolant hoses, thermostat outlet, all the bolts in the vicinity, wiring harness, a/c condensor and brackets, (rad?), a/c plumbing hoses, pulleys, fan (may run backwards to what you have from the donor van), fan shroud (important item for an a/c unit to have ample air flow through rad and condensor), carb fast idle dashpot and wiring, check if anything bolts up to an exhaust manifold bolt in case the depth is different from your donor van parts, etc. if/when dismantling remember there will be pressure and oil in the a/c system. it may be unlawful to release the refrigerant into the atmosphere, there could be inline filters and/or orifices in the hoses and/or connections so don't lose any of those parts because it will make it easier for the a/c guy if he has the originals to compare parts when he recharges the system. take some hose plugs with you to cap all the connections so dirt and excess moisture stays out and oil stays in, grab the a/c spec sticker or take a pic of it from the stock vehicle so the a/c tech will know how much refrigerant and oil will be required for the stock system. you can use the stock dryer etc for mock up but understand that the tech will replace some parts because he will want new filters and dryer. moisture in the old parts can become ice in the a/c system plumbing which causes blockages. if doing the donor a/c it is a good idea to take a cardboard cutout of your firewall and inner fender configuration to take with you just so you know what you have for available space. remember the inner cowl and the wiper motor and arms on the old truck are gonna also be something to consider for space limitations. when you factor in all these items you may be liking the tucked away vintage air system after all, lol.
https://www.summitracing.com/parts/vta-499193
serpentine belt systems on small block chevy, with pics. a good read if you plan a donor a/c system or simply wat a serpentine belt system.
https://67-72chevytrucks.com/vboard/...d.php?t=233335