my 8.1L was not any more of a gas guzzling turd than the 6.0 and had mad torque stock. Instead of adding the Allison though, keep the 4L80 as it can take pretty much whatever you care to throw at it. Allisons are good for stock applications unless fully built. Spend the money instead on a good CAI, Stainlessworks long tube headers and a good exhaust system, a mail-order or dyno tune, and 4.10s if it didn't come with them stock.