With it being a '70 truck, you may have the 4.11 gears already. Otherwise, you more than likely have the 4.56.
Depending on what model '70 it is, there is a possibility it could have a Spicer/Dana 60 differential, especially if it a leaf spring chassis (usually a Camper Special). 4.10 is the common gear there, but different ratios are very easy to find for it.
