I bought this 521 (1972 L16) a few weeks ago. After fixing the timing chain/carb and a few other things the truck ran fine, but on the freeway I was getting about 50MPH at 3000 RPM (4.375 Diff) (Confirmed with Tach Installed and my GPS set at MPH) I have a 4 speed tranny. I confirmed the ratio by lifting the rear of the truck and supported the rear of the truck by jack under the diff and two jack stands one for each wheel, and I also choked the front wheels, (dont want the truck to roll off the stands) so make sure you do the above when you lift the rear of the truck.
I marked the prop shaft and rotated the tire 1 revolution. The Prop rotated 2.25 turns for 1 Rev of the tire. So for 2 Revs of the tire, the prop turned 4.50 revs. When I took out the third member it was stamped (35:8) = 35/8 = 4.375 on the ring gear, so it was very close. One has to turn the tire 2 Revs to get the correct prop Revs, don't know why, but this is the way it is calculated
I found a 3.70 (37:10) Diff 3rd member (from a Fairlady) on eBay, ($160) and decided to give it a try, Calculated (RPM VS Speed Calc online) that I would get about 60MPH@3000RPM.
I decided to take the two 3rd members to Anthony at Hoopers in Sun Valley CA, a reputable Diff shop. I wasn't in the mood to fool with changing the end gears from the one to the other, even though it seemed quite easy having checked it done on youtube. Anthony charged me $150, and I'm glad I paid it, He said he had a very tough time getting out the pins and broke two punches in the process. He said it was a real workout!
I put it all together and now it seems so much nicer to run the engine at the lower RPM, and in fact the calculator was right, I got 10MPH more at the same RPM. Truck seems slower off the line now, and I guess I will have to use third gear going up grades. But on the strait away, it cruised a nice 60 MPH at 3000/3100RPM.
I was thinking of putting in a 5 speed, but I think I will stay put, don't think the L16 is strong enough for both diff(3.7) and tranny swap. I think its one or the other.
Thought I would share.