OK, series 3 carriers are for 3.42 and numerically lower gear sets. Series 4 are for 3.73 and numerically higher.
You CAN run 3.73 and numerically higher on a series 3 if you run a "thick" ring gear. The series 3 being intended for 3.42 and numerically lower mounts the ring gear further from the pinion due to the higher pinion tooth count in the numerically lower gear sets. Hence the pinion itself will be larger in dia. Gear manufacturers offer 3.73 and numerically higher gear sets with a "thick" ring gear to allow for operation with series 3 carriers. They are often designated as such.
I am running 3.73 Richmond with a "thick" ring gear on a series 3 carrier on my K-10.
You cannot run 3.42 and numerically lower on a series 4 carrier. Thin ring gears would probably not work out so well...(Which they do not offer)
And I agree with CaptainFab on everything he said, especially that it sounds like you have a kit for a car 12-bolt. Larger pinion shaft dia.... Need to get 12-bolt "truck" components.
You need a master rebuilt kit like either of these 2:
http://www.ringpinion.com/b2c/PartsL...D=192&CatID=27
And a gear set such as this that is specified with a "thick" ring gear:
http://www.ringpinion.com/b2c/Produc...nd_Pinion_Sets