I rub on hard flexing on the rear wheels. I think that Gen 1 must have higher wheel wells, and I have taller springs I am at 14.5 inches to bottom of frame with no BL. When I went from 30" tires with speedometer about 10% off to 33" tires my speed is right on, I am running an electronic speedometer from late model(?) GV. My rpm's are at factory 2600 at 60 mph. I don't know where your speedo is what your 60 mph rpm's are. but if your gear tire ratio's are going slower than speedo says then larger tires won't hurt as much as if your speedo is slow already. (I think they set the tire gearing ratio's for optimum efficiency.)
I have a larger hp gain to play with, you already had a hot rod. I did notice a big difference in performance and mpg when I went from Pro Comp "Extreme's" to Toyo Heavy Duty Mud Tires. Same size tire but much heavier.