To L+B the flywheel, you dont need the whole engine doing at the same time to see a benifit.
Its a bit like fitting a back box, you see a slight improvment, but do the whole exhaust and you get more benifit
Ive had clutch kits with a flywheel, and the cover is balanced on the flywheel and marked so you can fit in the right position on the car. You will get benifits from just this, but not as much as if you do the whole engine.
To do the whole engine, you give the engineers, the flywheel, clutch cover, crank and front pulley