8.75 rear diff,742 case,narrowed 3 in. HELP

I've came across a 8.75 rear diff,Drum to drum narrowed 3 in. (1 1/2 in on each side), 742case,brand new Richmond 430 gears, new spool, new bearings and seals, truck axles resplined by Moser after shortning,big yoke 30 splines. I am building a 65 barracuda abody. 340 motor around 400 hp. I want the widest rims and tires on rear of car without cutting body or fenders. Is this a good set up for this car. I want 18 x 9 rims on rear. may be 18 x 8. I know I'll have to move spring leafs in to get clearance without cutting car. Anybody done something like this on a car like this one, or just know what to do or not do on this. Is this a good set up I found or stay away from it. How about the truck axles being shortned and re splined? New to mopars need help. This rear diff is in a 68 barracuda now,