really need more information like what you guess the flywheel HP is and what the inecjtor part number is
If those are 42 lbs at 43 PSI rail pressure and LS1 uses around 60 PSI then those 42s are now like 50 plus lbs are you sure those are not too large since that is about 25% more flow then if a fuel rail of 43 PSI.

The PCM was designed to use a average pulse width time of around 3 mSec for part throttle. the longer on time gives the PCM better control and when you start forcing PCM to use shorter on times you put more load on CPU to keep spending clock time in recommanding injectors.

Remember the voltage offset is a delay time after PCM commands pulse width so if you set that too long or too short then injector could be coming on before or after ign coil has fired off the sparkplugs so I'd require the vendor who sold you these to supply what offset time should be.

I'd drive the injector flow values of IFR with larger values to drive it leaner and then see what the average pulse width time is.

A injector that was flowing 42 lbs with a 58 PSI rail pressure using the 80 % duty cycle rule could handle 540 flywheel HP so you can see if your injectors are really 50 lbs that would be
640 flywheel HP, so is your engine require that much injector flow ?