Classic/Antique Car Repair: Rear end ration, 700r4 transmission, ring and pinion

Question
I am installing a rear end from a 76 Monte Carlo, and a 454 motor from the same into a 48 chev.pu streetrod.

I am trying to identify the rear end ratio, the only #'s that I can find on the rearend are raised.  They are   GM18,  Z ,  F156 , and 1261602 .  

Can you be of any help in identifying this rear end.  

Also, I am under the belief that a lite pickup with a 454, a 700R4 transmission and a 3's rear end should get good gas milage, say 17 to 22. What do you think.  

Answer
JIM ;
    Hi  , well as far as the rear , I would think it is a 390 rear. If it is from a stock monte carlo than it should work good for you.It is a lsd  limited slip type rear. as  far AS the #s cant tell. you can count the teeth  on  the  ring  and  pinion to tell the gears   ok....