Perfect opportunity to do a 3/4 ton axle swap, and it will probably be cheaper. 4.10 gears are very common in 3/4 ton trucks and you should be able to source a decent pair of axles out of the junkyard for the same price or cheaper than buying new gears and the associated parts, and a complete axle swap is typically a lot easier than replacing just gears. The other benefits are a big upgrade in rear axle strength whether it is the SF 9.5" or FF 10.5" rear axle. If your son is anything like me when I was a kid the 10-bolt rear won't last long if you rebuild it.
The front axle is super easy to replace and doesn't even require you to open up the brake lines as you can re-use the same calipers. Probably a new set of u-bolts and that's it. Rear will require new u-bolts (they are different size...and make sure to get the spring plates off the donor rig) and a conversion u-joint. You will have to replace the 6-lug wheels with 8-lug wheels. If you keep the same tires you are limited to 15" 8-lug wheels which on one hand are very cheap, but on the other hand selection if very limited to a few different steel versions. For strictly looks I think most people who know anything about trucks and off-road will get a kick out of an 8-lug K5.
If you really, really want to stay with the 10-bolt rear axle you should be able to find 4.10 gears pretty easy. Probably won't find a rear 10-bolt with 4.10s unless you get lucky and happen upon one with aftermarket gears, but 4.10 geared front 10-bolts were very common on 3/4 ton trucks. The gears are the same front to rear for 10-bolts, and front 10-bolts use the same gears whether 3/4 or 1/2 ton. Actually buying used front gears likely means they have very low miles on them, assuming they didn't come out of a full time 4x4 truck from the late-70's. Just make sure the donor axle is a 10-bolt and not a Dana 44.
