I think the easiest and cheapest would be to pick up a set of 16" wheels and a decent used set of LT235/85R-16 tires for it. If you're running a 4.10 rear end,you'll be running about 3,110 rpm at 70 mph,that comes down to 2,900 rpm at 65 mph. You'd probably be into it under $200.00
Speed