I have become intrigued with the possibility of using 5.6 heads on my project due to the larger ports and intake valves. I bought a pair of 5.6 heads yesterday and dropped them on the block and they seem to fit just fine except the cam tower head bolt holes are off about 1/8”. A little machine work could fix that. All the other head bolts line up. All the important holes(coolant, oil) are close enough to work as is or a little match porting to line up.
One thing I noticed is the combustion chambers are large (compared even to the 4.5 heads) so that will drop the CR some. I did a super quick measuring job and came up with 50-55cc volume. The valves protrude past the head parting surface so getting the measuring plate to seal was a chore. i will need to make a custon plate with notches to clear the valves.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT