You should be able to do a key cycle and check for a code. Put the key in the ignition and turn it on (but not starting the truck) then off, then on, then off then on quickly and a code should pop up where your odometer is and see if it gives you a code (it will either say DONE and flash "-----" which means there are no codes, or it will give you a "Pxxxx" code, write that down and post it up. I have heard if the "check gauges" light comes on there is something else than just a low gauge. One guy had to replace his whole cluster to get it working again. I'll see if I cant track down the post on another site.
Basically though it is pointless to do anything else to your truck to make it go faster until you get the "Check Gauges" light to go away permenately. Check for codes using the key trick I posted up and then check your oil, your mechanic may have only put 5qts in, while the HEMI takes 7qts so that may solve the issue but if there are codes it might be something worse and much more $$$
AF - Damn that truck is slow, those big ass tires are just not doing anything for youeven with that cam in there. Your truck and Okla's look very close though, would be a closer race than you guys think.