better gearing/transmission for a '59 to do extended highway speeds?

[alden]

The maiden voyage of my 59 fleetside made it up to around 65 or so - didn't want to push it given it's deathtrap stage at the time - but I don't think it had a whole lot more, nor would I want to keep it at that speed for an extended period of time.

Right now it's got the stock straight six with the 3 speed and granny gear. Would a different rear end or alternate transmission be useful in helping it cruise at higher speeds and is there a possible side benefit of better mpg?

Thanks!