Ford Repair: Gear set, stock ring, stock gear

Question
HI,, i have a bronco 78' 351/400 engine 3 speed aut. trans.  and I put on 38's tires,,, the problem is I don't know what gear are the apropiate for this tires ,,because a mechanic told me if I put on a high gear like a 4.88 or 5.13 I will need to put on a locker too.. because this high gear are biggest than the stock gear set..

I want to know if it is true that of the diferent diameter between high ring and stock ring  and what gear set you recommend me for highway drive and a litle off road  fun !

note: the engine has  a aluminium intake manifold with a holley four barrel 600 cfm ,,, headers and  wires and coil Accel

Answer
Hello and thanks for the question...I'll try to help.
Ever hear of the term KISS....keep it simple stupid. I've never heard of tires making a difference in gearing. Keep it stock and if you want a more agressive rear end then you can change out the gears.
Erik