I have a 49 Dodge truck on a Dakota chassis build here on Moparts. I've driven my truck through a winter (north west corner of IL) with actual snowfall, frosted glass, and 0 degree weather, my truck sits outside when not being driven.

Your 56 should get pretty close to the same mpg as the Dakota got, if you keep the EFI. The shape of the body won't have much effect unless your truck only sees lots of highway miles, then a slight difference due to aerodynamics may make a difference of 1-2 mpg. My 49 with a 5.2 Magnum and a 46RE and 3:55 gears on a 94 Dakota 4x4 chassis got just under 18 mpg @ 75 mph with 235 x 75- 15 tires. Around town is 10 - 13 mpg. The 56 cab is more aerodynamic then the 49 is.

I would strongly suggest that you add sound deadener/insolation to the inside surfaces of the cab (in addition to door & kick panels, and a headliner), especially the roof, firewall, back of the cab, and floors. That stuff makes a huge difference in keeping the heat in during the winter, and keeping heat out during the winter. I use the self stick stuff made from butyl rather then the stuff made from asphalt, the asphalt stuff can melt if the temps get over 90 degrees during the summer. I used stuff called NOICO, but since it came from Russia, it may not be available now, but you should be able to find alternatives.

You also need to be sure your wipers are up to the task, if your 56 still has the vacuum wipers, they are not going to cut it. You will need well functioning electric wipers, and a windshield washer would be a great addition as well.

When you are hijacking stuff from your Dakota donor, be sure you get the heat/ac box and the defroster duct work for the Dakota and adapt it into the 56 dash, The 56 defrosting system needs a lot of help. The Dakota heater & defroster duct work clears the windshield pretty fast, keeps it clear, and warms the cab in about a mile with a 195 stat. Its a cold ride before that 1st mile. Heated seats would be a great option... Gene