I should have mentioned my 2-speed rear swap (6.14/8.36) was on a Eaton rear, not a GM rear like you have. The Eaton rears have a better selection of gear ratios.
Is your 350 sbc a car or truck engine? Do you know what HP the engine is? Do you have a governor on it?
With 9.00x20 rear tires at 40" tall, 7.20 rear gear ratio and 3700 rpm you should be doing about 61 mph...sound like you need to wind that small block up and go! Your governor (if you have one) is most likely set at 4000 rpm. If 53 mph is all the engine will do (confirmed with GPS), you need to revisit the engine for lack of power issues.
Mike B
