I just removed the carpet in my 60 and what surprised me the most is the amount of heat that comes off of the transmission / transfer case. Have you thought of using some asphalt adhesive insulation that's got reflective film on one side? Maybe stuck to the bottom of the tub ( reflective side down ) will reflect a lot of the drive train heat down away from the cab. A downside to that would be that water / mud / whatever could get caught in the stuff, but if you applied it with a heat gun, you could probably get it on there tight enough so it wasn't an issue, especially with an aluminum tub.
Just throwing some thoughts out there. In my truck, I thought it'd be good for warming the truck up faster, but I'm sure a 60 is significantly different than a 40 in this regard