If the B20 is a '74 or '75 (flywheel held on with eight bolts), B21 pistons will swap right in. If it's a '69-'73 B20 (six bolt flywheel), there's a mismatch in the wrist pin size, 22mm diameter vs. 24mm for the B21. What you can do in that case is bore the rods for .0006" running clearance and install the pins without bushings. Sounds iffy, but works just fine.