Build the trans?...or no

[kTm520]

I got an 05 with 85,000 on it. Arps, egr delete, powermax turbo, custom tunes from vivian, and some other little stuff...would love to have a nice built trans to really throw the power through the truck and shift fast. Stock tranny is fine but for some reason when i get on it it gets a little hung up and a little longer to shift between 3rd and 4th. Id buy used? New? i dont know to much about transmissions im a body guy and really dont wanna spend to much money but i have some so i figured before i blow it on something else maybe ill blow it on this. so any help would be great.