Chevrolet Repair: Milage, speedometer calibration, k5 blazer

Question
I have had a new 350 eng. installed in my 1984 K5 Blazer replacing origional equipment 305. My milage is only 10 miles per gal. I realize I need to drive it more and get it more nearly broken in, but I am wondering if the carb from the old eng. needs rejetting. I also got a ticket for doing 76MPH when I was indicating only 70, the transfer case had been replaced prior to me getting the Vehicle. Do I replace the speedometer drive gear or the driven gear? The State Trooper told me that they would probably forgive the speeding ticket if I got the speedometer fixed.  Thanks, Frank

Answer
What was used or deleted when engine changed, such as computer, different intake manifold, headers, exhaust, and, how was the 350 built? Such as compression, camshaft style, ignition, heads(valve size), cc of combustion chamber? Many good transmission shops can repair speedos, or look in your yellow pages for speedometer calibration shops. Also, if you have a different size tire other than stock, of course the speedo will be off, and will need to be calibrated to that size of tire.