Yes you had to change to the Flex fuel injectors, they are larger and the flex fuel computer expects the larger injectors.
p1121 is throttle sensor readings not consistent with Air flow(MAF sensor)
Throttle position sensor(TPS) is a pretty simple device, it is like a light dimmer or volume control, it gets 5 volts from computer and sends back a variable voltage from .6v to 4.8v depending on throttle position.
When throttle is closed(foot off the gas pedal), TPS will send computer under 1v, .9 is spec
When throttle is wide open(foot to the floor) TPS will send above 4.5v
When you push down on the gas pedal the throttle plate opens and more air will be allowed into the engine.
What p1121 means is that the Air Flow(measured by MAF sensor) isn't changing or responding as expected.
Are you using original '99 upper intake with '99 TPS?
Could TPS have been damaged?
Bad TPS could cause lack of power and limited speed, as the computer "thinks" you are never putting gas pedal down all the way it may be topping out at 3volts, and it would explain why MAF data and TPS don't match.
P0171 is Lean on passenger side of engine, this is most likely related to above.
You would expect P0174, lean on drivers side bank, with it but it may just be pending.
After fixing p1121, clear codes and see if p0171 returns, if so check lower intake and fuel rail on passenger side for leaks.