I average ~ 16mpg in my 00 Ram with a 24V and 4.10 rear end. You'd get near 20 with a 12V & 3.55 rear end. I haven't looked at what mods you would need to get the thing installed.

I liked my Burban but wanted a truck so sold it when the engine went out. My advice would be to simply replace what was there. Messing with something the wife needs to rely on is frequently problematic. Mess with your toys & make them a family DD when/if finished.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.