Do some research on the driveshaft.They don't much like being drove at high speeds,as the RCSB 2wd's with the 8 speed have a tendancy to throw driveshafts,especially the 3.92 geared trucks.It can get very expensive if it does chuck the driveshaft out from under itself,as quite often it breaks the transmission,and there's even been a few RCSB 8 speed trucks catch fire and burn to the ground if the shaft goes through the fuel tank,or hot transmission fluid hits the exhaust and catches fire.
Critical driveshaft rpm is right around 5400 rpm,so when you get to about 5,000 rpm in 6th gear,you might want to start backing off the skinny pedal.
6th gear is the direct gear,so what the tach is reading in 6th is the same rpm the driveshaft is seeing
The 8 speed 2 wheel drive Regular Cab Shortbox trucks,have a very poorly designed driveshaft,they use an unsupported slip yoke in the driveshaft,and at higher driveshaft rpms,the driveshaft basically starts to turn into a skipping rope.
Nice little toy though

Your picture of the bottom side of the truck actually shows the unsupported slip joint,it's under the rubber bellows that are about 1/2 down the driveshaft