MISERABLE Mileage.............

did you make SURE that the manifolds aren't leaking? Did you plane them out or mill? My super six had some vac leaks initially and it was running ok but I had it tuned to run with the leak and yeah it got about 8-9 MPG. after truing up everything ( see this post http://www.forabodiesonly.com/mopar/showthread.php?t=33620 )
I'm getting 15ish around town and 21-22 on the hwy.

oh and keep a close eye on the 160* tstat, It's great if the system is up to the task but it may end up overheating....
I know, you are thinking I'm crazy but it's because of:
1. tstat opens at 160
2. coolant flows from the rad into the block and from the block to the rad.
3. "cool" coolant hits the tstat and it closes.
*4. if the rad can't cool the coolant enough before it makes it back to the tstat, the tstat will never close and the engine will get hotter and hotter.