My whole intention was to use dead space to make it functional - hence not wanting the hot engine bay. There's room ABOVE the spare for a five gallon tank, so I'd say use that and save the spare space for whatever else you might want in the future. Frame rails worked great for my compressors, as long as you route the air intake up higher so that they don't suck in gunk. Mine have been there for years with zero problems. I do occasionally have random leaks - same with my Jeep that has a motor drive york compressor, but the leaks are never the compressors. They are usually fittings. Onboard air can be annoying, it seems like I have to chase random demons in each vehicle every year, but that's just life. One suggestion I have - use these chucks, they are awesome:
For hard mounting a connection, like through metal:
https://www.summitracing.com/parts/arb-0740111
For direct screw in to an air tank:
https://www.summitracing.com/parts/arb-0740112
Little dust cover for either:
https://www.summitracing.com/parts/arb-0740113
I've had way too many problems over time with the cheap generic ones from Home Depot or wherever, and these ARB ones are much nicer to operate.
Hopefully it's obvious, but you do want to use a relay in this situation since those compressors draw a lot of amperage. Your switch should just control a relay to provide battery power straight to the compressors. If I recall, my specific compressors have that relay built in, so I have a small control wire running to them, plus big fused battery wires. I want to say they're 40amp each. It's been a while since I messed with them, so I think I have them dialed in. Also make sure that you have some sort of rubber isolators on your compressors, don't mount straight to metal or they will be very loud.
Just looked it up and these are the compressors I installed back in 2016:
https://www.amazon.com/gp/product/B015NS9LW6/?tag=fordraptorforum-20
I used an air tank I had sitting around, I think it's from a semi. It doesn't have feet, which makes fitment nice, but the mounting involved some creativity. Make sure to use a pressure switch and a blow off valve in your installation.