Ramcharger exhaust 2-1/4 duals or single 3"

So, I do have to pass the sniffer test in Delaware. Last inspection I failed the emissions test with a 601ppm HC ( needing <600) I kept trying, but without cats, they refused to pass me. I got a pair off Ebay, welded them in and passed emissions. This was with a worn out 318 and the original carb that was untunable. It now has a fresh stock 89, roller cammed 318, cast spreadbore intake and a eddy 1406, HEI ignition, and the exhaust as mentioned. The 1406 has gotten me the best mileage (and drivability) so far. I tried 2 different spreadbore Holleys, a square bore and a thermoquad. I still have a factory quadrajet to still try. The eddy has been good. I make a change and run 2 tanks of fuel to see the results. I use the same gas station and my route to and from work is unchanged.
I never thought of the front hubs at all. I thought that they were disengaged when in 2wd. I will look into the manual hubs especially if it will give me an advantage of mpg and I rarely use 4wd in Delaware. It will be easy enough to turn the hubs before pulling out of the garage if it ever snows here.
Thanks everyone!