Show-n-go5.3

I just rebuilt this transmission to repair a worn out high milage trans and build to hold 1000hp. I took the truck for a little drive and it seems to shift fine ( not as hard as i anticipated) but the trans temps are up to 180 with just stop n go driving. When i got home i felt the cooler lines an they were still cold. Whats ur thoughts? How do i confirm the dip stick is reading right (aftermarket Lokar). I installed a new Sonax PR/ boost valve and the pump didn't appear to be worn, although i didn't check the face clearance with a precision straight edge. Im thinking this problem existed before the rebuild because i didn't have much fluid in the cooler when i took it out. What are the possible causes of this problem?