I would keep it as is. Just get a http://www.nelsonperformance.com/ tune, and look at this: http://www.ls1truck.com/forums/front...in-budget.html
But to answer your questions, remove the 5.3, swap all accessories onto the 5.7 and drop the 5.7 back in place of 5.3. Yes, you can use the truck intake to get it back on the road and later do a swap if you want to. I think you can just order the 5.3 truck headers and you should be fine. And yes, the computer will need to be tuned. As far as the injectors go, not sure someone else will have to answer that. Hang on to the LS1 for a different project.