Without measuring I can tell you that a B30 flywheel is about an inch bigger
in diameter than the B20, and the clutch likewise. The input shaft is
bigger and (I think) longer. Both are standard dimensions, however
and I believe some Chevys will use the same clutch as the B30.
Naturally you could bolt a B20 flywheel and clutch up but the input
shaft would not go through the B20 clutch plate. Worse, the ring gear
doesn't stick out far enough to engage the starter so you'll have to
push start it every time. (You could reduce weight somewhat by leaving
the starter off......)

The best bet, if he INSISTS on a lighter flywheel (which will give better
acceleration but NOT more horsepower) why doesn't he just take it to a
machine shop and get some machined off the face? While I was
in Panama and installing a Chevy clutch that was more than 3/4" too thick
to clear, I took 1/8" off the front face as a start to being able to bolt
it together. I also had to reverse the throwout fork to get the pinch
bolt on the back side, put a 1/8" shim between the bellhousing and the block
and do some trimming of the clutch adjusting studs and miscellaneous parts
inside the bellhousing including the throwout shaft bosses. It was one
solid clutch and eventually the crank arm that was welded onto the throwout
shaft started moving independently as the weld began to fail.
I'm back to diaphragm clutches now and they are OK. I sold my lightened
flywheel to somebody who was hurting for any old B30 flywheel a year or so
ago. Don't remember who got it.

E-mail me if you REALLY need dims - I have one in the garage.
--
George Downs Bartlesville, Heart of the USA!