Hi Doug
I am no expert, but I have had a LOT of luck with 120mm bloodworm wrigglers on 3/8oz 3/0 jig head.
So much so that these are all I use for flatties now.
The weight is on he heavy side but it will gauarantee you remain on the bottom.
Also if you are worried that the 120mm is too big, i have caught flatties on them smaller than the plastic itself.
Good luck!
cheers
Paul