Are you increasing the stroke as well? If not, and sticking with a 4" stroke, id just use a ext balance GM forged crank. But if youre going 4.25, and it comes setup for nuetral balance, id keep it that way. Whatever you spend doing balancing work to modify the new setup, will help pay for a new flywheel setup and damper. Nuetral balance is superior imo.

As far as your carb issue, Id look into a blow thru carb from Roger at c&s. I think Mike Tkach would agree

FYI, when i upgraded motors from 4 inch stroke to 4.25 , i bought rotating assembly,. motors were internally balanced and i keep them that way.
think crank is scat,,, rods eagles,,, pistons srp's...

Port engine is stock 420 Merc setup ext balance. Stbd engine is internal balance from a previous failure. We snapped a crank in 2 pieces 7 seasons ago on the internal balance engine. It was an eagle 4340 4" stroke crank. I believe internal balance is the only way to go when using non stock combinations but I must stress that execution in bearing fitment and balance are crucial. At 4" stroke either way is acceptable just make sure whoever does your balancing knows their chit.

Lunati Voodoo is a nice crank at a reasonable cost.