If you go with an overdrive transmission, make sure the OD gear is a good match for your rear axle gear. A good running inline six can handle the OD if the numbers are right but will bog the engine for lack of HP if you get it wrong.
Sorry, cannot be done. I think you need to sell it to me.
No, seriously I have driven a '63 and a 68 before that one for many years on the interstate daily. They've both been long gone for more than a couple decades, but the speed limits haven't changed any since then. As long as it's in good mechanical condition, I wouldn't worry one bit. Upgrades can always be done later.