I'm not knocking the 4L60, but you would need to change from 6V to 12V (see Tech Tips for how to) then buy a transmission controller (computer in a box) which requires you to link up with a laptop computer (or handheld device with APP) and make setup changes yourself (trial and error procedure) in the software provided. You also need a new radiator with transmission fluid cooler or a stand-alone cooler. At this point you have already broken the "originality ice" so you might as well replace the carburetor with fuel injection including electrical fuel pump and a gas tank with recycle connection. They make a box that controls both transmission and EFI. The original speedometer is mechanical so you would need to buy a mechanical to electrical converter. If you love computers and programming; this is the way to go. As is with all after-market electronic kits, they are subject to have unforeseen bugs are failures because the manufactures do not have the resources to stress test like GM, Ford, Mopar and other major automobile manufacturers.
A good rebuilt TH350 transmission, with adapter kit for a 261, does not require any electrical changes but the transmission final gear is 1:1 so unfortunately the is no OD. A rear axle gear change and taller tires will help but you don't have that wide range that 1st to 5th gears give you with the T5 BUT the wife is happier.
A compromise would be to buy a used modern truck, as a second truck, that has all options that you want, so your and wife are happy can be happy.
