The Arctic Fox
I'm Awesome
You guys are on the right track with the Topkick dash, but its even easier than that. Some of the TKs that had A/C delete have a pocket in the middle of the dash where the aftermarket radio would go. And its about the right size for a DD. So you dont need to replace the dash, just the middle insert. Youll lose the vents, but it would be fairly easy to re-rout the air up to where the OEM radio was and maybe use a vent of 73-87 CK.
On the the most basic model gmt400 the oem radio slot, I think, has air already running to that spot from factory. This is only on the trucks without radios. I'm probably wrong but I thought I saw one like that 6 years ago when I was working in a mechanic shop.