I am a bit late to this thread but a few comments from my 3.8 conversion in my gen II.
MPG, in general I see mid 18s which drop to mid 17s driving in the Adirondacks. When I get to do a day on the highway and can keep the speed up I get in the low 20s for mileage.
The brakes really need good pads, the ceramic jokes went away on one heavy deceleration and the grip has not returned. Hawk HPS are going in.
I only have a small amount of towing with this rig so far, to me the transmission ratios are spaced way to wide in the top gears and the torque converter lockup is there just to raise my blood pressure. The six speed is sitting in my shop waiting for the adapter to be made. Once it is in this should be a nice rig.
The engine makes power, good power. Most is lost in the transmission but when I go to pass someone this truck has no problems, in a normal pass I back off once I am next to the other vehicle, if a stay on it till I am clearing his front bumper I am clearing 80 mph. I like being there but it is frowned upon by some. So the 40 to 80+ pull is strong.
How this relates to the factory 3.8 I do not know. They run taller gears and a different automatic. I can not imagine the tall gears to be of any value but I hope the new 5 speed transmission is better than what came in the Gen IIs.