Thats where it starts to get expensive.
Got a good deal on an 8.1L engine, but now you need all the little things to make it actually work. You needed to get everything else out of that truck you got the engine.
But since you didn't heres what i think will work..
You need a green and blue plug computer out of an 03-04 chevy truck and have it programmed with 8.1L parameters. If you can get a factory harness, and send it out to have it reworked it will save you some bucks compared to buying a whole new harness.
You will need a pedal assembly and a couple of modules to make the drive by wire work..
I'll have to do some more research but you might be able to use any genIII (4.8/5.3/6.0) truck harness on your 8.1L. they are very similar.
I need to look at a few things, but if thats the case, I might be able to set you up with a factory harness that i rework for 4 wire hookup in any vehicle, pedal, and computer package ready to rock on your engine, everything your missing. PM me if your interested.