The normal way of injector size compensation is, to trick the ECU into reducing pulsewidth by altering the MAP or MAF input signals.Since your application requires a pretty significant decrease in injector pulse width, The only way I can think of is to alter the ECU itself.The ECU uses a multiplier to calculate injector pulse, you would simply adjust that value, your ECU's microcontroller will need to have external memory for it to be easily altered.
For the voltage question, you could use a Zener diode or a voltage regulator to keep at 4.9 volts, the latter would be the prefered way.