The tires you're running are a bit undersize for your truck. The base size for the '55-'59 3100/3200 was 6.70-15. According to the specs I've looked up on the tire dimensions, the tires you have are about 1 1/4" smaller than the stock size, the 6.40-15s being approximatly 26.88" and the 6.70-15s being approximatly 28". Therw will be some varaince depending on manufacturer/tread design/inflation pressure. With the OE diameter tires and 3.9 rearend ratio, it looks like you would be turning a bit over 2600 @ 55 mph.
