I've got a 3.9 NV2500, 3.90 8-1/4 that's going to be upgraded sometime... but I love it the way it is too much right now! I've converted it to manual steering and electric fan, and it can do a 120 mile round trip on less than 6 gallons of fuel throught the mountains and a 1,000 pound load on the way back... no need to mess with perfection yet!