SFI does not make any more power than batch or TBI.
I'll say again.
SFI does NOT make any more power than batch fire or TBI.
SFI, in stock form is only enabled below 3000 RPM, where it reverts back to batch fire, since there isn't enough time to fire the injectors sequentialy at around the 3000 RPM range, and supply enough fuel.
SFI is only used for emissions, and lower fuel consumption.
The 3.4 SFI intake is terrible for flow. The "plenum" is tiny, because the casting is thick, si it'd not a "great" intake.
You also need much more than just the harness from the F-body, you will also need the ECM, but I would expect that to be assumed as part of the "harness", and either the stock VATs module and matching ignition switch, or resistor value. Another option would be to construct a module that mimicks the connection between the VATs module and PCM, there are plans for it on the interweebnetz. I also believe that you may be able to buy the module already constructed.
If I wanted to change from TBI to a multi point injection system I would use a '7730 or '7749 ECM, and a matching program to the way the engine will be used (S_AUJP for N/A, code59 for forced induction), and run in batch fire mode. There is lots of documentation for these ECMs and programs for tuning and other modifications. The 3.4 SFI PCM has not been hacked, and has just about no tuning support available.