The 49-53 Merc clutch has 6 evenly spaced bolt-holes on an 11-5/8 bolt circle. It's the same bolt pattern as used on a common Chevy flywheel, so there is room for confusion. One identifying feature of the Merc pressure plate... If you take the palm of your hand, and press on the fingers, you can move them with palm pressure. This is on a 'loose' pressure plate, laying on the bench. Can't do this on most other clutches, especially, you can't do this if you have a Chevy pressure plate.
Hope this helps. Your flywheel is correct at 11-5/8, and the 10-5/8 pressure plate is incorrect.