If it's a manual transmission then it won't be a problem, it's the autos that grenade on a regular basis. Which V-6, pushrod or SOHC? It will make a difference in the millage. How wide are the tires? 10.5 or 12.5, the wider they are the worse the millage is. Most importantly what's the differential ratio? If it's stock, 3.27, 3.55 something like that the millage will suck. If it's 4.56 it should do alright. Did he swap a D-35 and 8.8 in to replace the stock axles (D-27 and 8.0) if not, the front axle is weak for those size tires.
If you do it right a lifted truck will make near the mpg's of a stock one. My truck on 34's averages 13 around town and 18 on the high way.
BUT, if you have to ask this question, then you probably shouldn't be looking for a lifted truck.