No you don't necessarily need a new flexplate if you've got a 6.0. I didn't like the idea of opening up the bolt holes and I already had a Gen 1 torque converter laying around so I bought a new flexplate from TCI (Part number 399753). It was $215 from Jegs, which I didnt think was too bad.
But since you have a 6.0 you should already have the flat flexplate and adapter to mate to a Gen 1 trans as those engines came on 4L80E transmissions, which is essentially a Gen 1 trans with electronics.
Jason W.