It's probably 4.55:1 if it's stock.
If it's a 4 speed, I'm a little surprised that you can start out in third. (Very surprised if it's a 3 speed). IIRC, the gear ratios on an SM420 are as follows:
1) 7.06:1
2) 3.58:1
3) 1.71:1
4) 1:1
R) 6.78:1
That, coupled with a 4.55:1 in the rear would be about right for 50-55 mph, depending upon wheel and tire size.
Here\'s the tool I used . I used the 235/75R15's that I have on my '46 as a reference; I don't know what size tires your truck is sporting.
If you can find a diff carrier from a 67-72 3/4 or 1 ton pickup truck (you don't need the whole pumpkin), you can swap it out for the one you've got now. You'll need to find an automatic, and then you'll only probably score a 4:10 diff (About 55-60 MPH). If you're REALLY lucky, you might be able to get a 3.90:1 carrier, but these are VERY hard to find. If you did manage to score one, you'd get 60-65MPH.
Just remember that you need to be able to stop. Huck brakes work fine (as do bendix) so long as they're adjusted correctly, but the faster you go, the longer it takes you to stop. Even at 50 MPH, the force that your brakes exert doesn't 100% counteract the momentum that your truck (mass times velocity) has.
