I'll try and kill your 2 birds here. To lay your dime out on a 20 you will have to relocate the blower motor if you want to keep the heater. I've seen guys make a fiberglass new housing for the blower motor but the easiest thing to do would be to remove it. You will have to tub the firewall. You will have to relocate the clutch master if it has one. If you are running 2 compressors, I would use a continous duty solenoid instead of relays. Just like electrical parts on your truck, you need to regulate the power going to all electrical components. Here's a link to help you with the install on the compressors.
http://www.fbirides.com/assets/files/install_files/3495.pdf
Hope that helps.