A C30 will more likely have a 5.14 gear, which is a low gear, not high (higher the #, lower the gear). 4.56 was used in the 3/4 tons, though it did become an option on the later 1 tons. About the highest you can get for that diff is a 4.10, using a center chunk from a '68-'72 3/4 ton. If you want a higher gear, like a 3.42, you'll need to swap out to a later diff, like a GM corporate 14 bolt 10.5" ring gear, or possibly the Dana/Spicer 70. If you're looking to keep it manual, and want an overdrive, consider either the NV4500 or a Ranger or Gear Vendors add on overdrive.
BTW-I only get about 12 mpg, IF I keep my size 14 out of it!!!
