I'm assumming your truck has 5.13 gears. Even moving up to a 4.57 gearset, that are plentiful in the old 'bolts will drop your RPMs about 350. Which isn't bad. Going with a larger tire, will help even more. Swapping out to a 4.11 will give you 500-600 more RPM.
Converting to a 6 lug setup in a 3800 truck would be more work than it's worth. The frontend is completely different than the 1/2 trucks. Not to mention, the rearend is not exactly a bolt in either.
I would just upgrade to a 4.57 or even 4.11 gearset.
