Find a rear axle center section from a 3/4 ton truck with the 4.56:1 gear ratio. That will raise your highway speed noticeably without having to do major modifications. The 261 engine will increase the load-pulling capacity, but not necessarily the top speed, unless you do something to raise the RPM on the top end- - - - -high compression, radical cam, dual carbs, dual exhaust, etc. That gets expensive in a hurry, and it's definitely not "stock" anymore.
Why not just put in the higher rear end gear, and keep the 216? It will still have enough power to run at highway speed, especially if you're not planning to haul 1-ton payloads
Jerry
