[Model51]

Almost all 1935 and 1936 1 1/2 ton Ford trucks came with the 6:60:1 rear axle ratio even though the 5.14:1 is shown in the period literature as the "standard". The reason was that the lower ratio provided a more realistic use of the meager 85 horsepower engine trying to move a heavily loaded truck. I have a Ford factory letter to all Ford Truck dealers in 1935, strongly encouraging salesmen to push the 6:60:1 rear end for "customer satisfaction".
I have taken apart many rear ends of these trucks and found the 5:14:1 only in the 131 1/2" wheel base panel trucks. I think the reason is that they were utilized for lighter loads and "in-town" deliveries. Unfortunately for us today, they didn't make many of these trucks so the potential supply is very limited. It took me 15 years to find one usable set of the gears and it was through a garage in California closing up and selling off all the "junk" in the back shelves. My only suggestion is to call anyone you can think of and be persistent. Your best bet is to find a 131 1/2" panel truck that's beyond restoration and buy it for the rear axle and hopefully it will have the higher ratio gears.
Another option is to install an auxiliary transmission in place of the short drive shaft directly behind the transmission while keeping your 4 speed transmission and 6:60:1 rear. These came in several variations, but some have a HI and LO range providing overdrive, pass-through, and under drive (12 different forward gears).