well any machine shop worth their salt. rods, pistons are obviously wieghted, and balanced across all cylinders but the crank will be balanced and counterbalanced to the stock pistons and rods in some case, unless identical pistons upsetting the balance already can cause problems. however getting a fully dynamic bottom end balance is worth doing if you intending for the lump to hold together for a duration this includes getting the flywheel, clutch cover, crank, front pulley and possibly the rods and pistons. this also depends whether the crank is naturally balanced or if its counterbalance by the rods and pistons. some are some aren't. i dont know about the YB. My V6 isn't naturally balnced and is counter balanced by the rods and pistons. speak to someone with good experience with the YB and see exactly what you can get away with. 4pots when out of balance it's quite apparent unfortunately, where as my old rattly 60deg. V6 you can actually get away with a quite a bit.

However getting it balanced will make for a very smoth revving engine.

Rob