[piecesparts]

So, you went from a Nissan that did not run to a truck that runs, but gobbles gas like it is going out of style. Gas mileage improvement was zero or negative in this case. The fact that you can drive down the road is a plus. The old Camaro engine must have something seriously wrong with the way it is set up in the truck to get that low of a mileage number. I would check timing and the basic items to make sure there is nothing wrong there. As for a tranny, a six speed would be better, but the install could be a major issue if you do not have the right background or equipment. The IROC engine install, is it a complete install with all of the IROC parts, or is it just the block with the truck's components on it. (carb, etc...) If the engine is trying to run on the computer then there could be issues with how the computer was connected and the sensors are reading the exhaust and coolant temp.