any moving part goes in when I have rotating assm balanced. Couplers, drive plate, flywheel, HB, etc. With a good balance job you should be able to set your beer on your flame arrester while checking your timing at full advance.

I used my stock Merc flywheel with the Eagle assembly and my engine guy simply milled off the weights on the flywheel when it was all balanced.

I used the 6.0 rods.

No complaints or issues.

There are 350 style balance which use the weighted rear flywheel and the front unweighted damper. They are PARTLY internally balanced. Or there are completely externally balanced cranks that are normally called 400 style. Both types are finish balanced.
You can buy a lot of gas and carb tuning for the cost of setting that efi up . I'd go carburetor and plan to learn how it all works. Once you go through one they start to make sense.
Or if you just don't want to get involved you could get an aftermarket EFI, they are designed for DIY. Probably around $2K though.

The problem with that efi on a 383 is it is open loop. It knows how far the throttle is open, and knows how much engine vacuum, called MAP these days. It has no idea it is feeding fuel to a larger cylinder than previously. So if you run it as-is it will be too lean and hurt the engine.