Get him to bring you back a pair of 750 holley blower carbs.
Re 600 carbs and sec metering plates: the plate once correctly tuning for your motor on the dyno will work just as well as a sec metering block.
no need to turn the carbs sidways. just get two metering plates and
modify for screw in holley jets (simple drill and tap threads) and dial them both, in on the dyno.
The 750 blower specific carbs will get you very very close to max performance, out of the box. Any required tuning/jetting will be minor ...The EFi is going to be a big water of time and money.
Water meth injection is much more effective when injected above the blower.
When below the blower you would have to account for the rise in manifold pressure under boost and compensate in the injection pressure
same with the EFI. Boot referenced fuel pressure.
When the fuel and or water/meth is injected below the blower, the blower is not cooled by the fuel.
The air is not cooled (charge air intercooled) near as effectively. Two big reasons why:
The 750 blower carbs will do a whole lot better job on this motor , than EFI.
They will make more power too. Thats 3 big big reasons, for a fraction of the price.
if you're going to stick with the 600's and want to optiminze them they will work best if you cut the choke horn off, mod the body for boost referenced power valves and mod for swappable screw in air bleeds and optimize the primary metering blocks.
(idle feed restriction, (power valve channel restriction size)
I would not bother. just get the right 750 blower carbs. Those you can mount sideways.