mix atf with gl4?

There were more traffic deaths those years for many reasons. Roads weren't as good, cars weren't as safe are the main two. No interstates or good straight roads in most areas, so there wasn't a lot of high speed cruising. That doesn't mean people weren't driving too fast on twisty roads when they shouldn't have been... just like they still do toady! But we have interstates and can cruise all day at 60-70 mph for 4-5 hours without stopping, then stop for gas and bathroom break and continue for another 4-5 hours. Not uncommon to take a 12-16 hour trip and get halfway across the country. Wouldn't have got half that far in that amount of time in the 50s. 

Cars of the 30's and 40's were not uncommon with 3.7+X:1 rear axles and 4.11:1 with O.D. But keep in mind, there was a national 35 mph speed limit during WWII and the travel was largely dirt road or 2 or 3 lane paved with center being the passing lane. The also it must be noticed that wheel and tire diameter was larger with balloon tires just making an impact. The advent of the automatic transmission with a torque converter to multiply the ability to climb steep hills slowly and move from a dead stop and the some time usefulness of semi-automatic transmissions made it easier to use taller more economical rear axle gears. The engineering to move from flat head to over head valve short stroke engines drove higher engine speeds which also allowed for taller gears to run down the roads which starting in the 50's became national freeway divided lane 4 lane roads. Lower diameter tires, 14 inches in the 1950's also helped. 4 speed transmissions became acceptable by the 1970'sand they morphed into overdrives by either incorporating a 5th gear or making 4th .7:1 with 3rd 1;1 giving the equivelent of a wide ratio 3 speed with an over drive.
Shifting went from a necesity to a skill untill automatics became better. Chevy 2 speed power glide, Mopar and Ford had their's too. Buick Dynaflow was one speed with a switched vane torq converter. Packards 2 speed with a lock up converter. All morphed into todays multiple speed boxes with lock up converters.
I learned to drive on a crash box, all gears were non synchromesh and shifting it was an art with matching engine speed to road speed by allowing it to drop in neutral between gears and raising it down shifting. When you were good at it, you could shift with out a clutch. And I still can.
And if you do not pay attention to the final drive of a 5speed conversion you have the equivelent of a 1948 work truck. A granny low, a wide ratio 3 speed and an O.d. A wholly unsatisfactory result after spending money to replace a good transmission with something un-usable. I shoot for 32 mph / 1000 rpm in the final gear and it generally gives me a satisfactory road geared car with a 5 speed conversion.
But that is my preference.
If I wanted to drive a 1948 work truck I know where to buy one from.

I think he just hit the wrong key.

Borg-Warner made a R-10 and R-11 overdrive, the lighter duty R-10 would be on a T-96.

It's been a long time since I drove a overdrive equipped car, but one think I remember. On a car with overdrive, if you had the overdrive engaged, but were below the engagement speed of about 28 mph, the car was freewheeling when you let off the gas, you could shift into low without double clutching and you'd get no gear clash, as long as you didn't rush the downshift.

That was another reason those overdrives were so popular back in the 50's.   We lived in a very mountainous area of Va.   When I was young my Dad, had both a 53 and 55 Fords, both with od. Mainly because he could get them with the steep rear end ratios, for the mountains, but still have a car that was decent for long trips.