This should be easy to nail down.
If this ran in the 57 then it should run in your truck unless :
1. The computer was swapped and its not the one that was running that engine in the 57.
2. There was a stand alone VATS unit that you didn't get. (iirc early models could be bypassed like that)
3. The guy never had the check engine light in the 57 or didn't care about the codes.
Fuel pressure check, a temp hot wire to fuel pump and a noid light will tell you if the Injectors are being shut down by the VATS or just low on pressure.
Since you have a security code, is suspect VATS, and the other codes is suspect a different computer.
This could have happened:
Hey man I just sold my engine out of the 57.
Really, didn't you have a custom tune on that computer?
Sure did, cost me a few hundred bucks too!
How about you swap me this computer and give me the one out if the 57?
Sure, here you go.
Get the picture?
swap : 88 k5 Blazer w/ 2004 5.3 & 4l60e, 212/218 cam
CAI, shortie headers, TCI 2600 stall, transgo shift kit,
Nelson tuned