BFG's are my favorite truck tires also, I've done a lot of 4 wheeling with them in the CO mountains. They will be stiff given the sidewall (10 ply?) but very durable. Although your speedometer will change, it might change back to being correct depending on what that previous owner did. I've gone up and down in size (2008 Tacoma), and the smaller tires, all else being equal, are slightly more comfortable due to the decrease in unsprung weight. I also agree that airbags will make the biggest difference (other than a new vehicle).