Thanks Frank-that's good info. I keep drifting off to thinking about trying to use 5.6 heads but I need to stick to my original plan of a simple to build engine. If I use alum block heads then my 3.5-4.5 accessories won’t fit (alternator, PS pump, etc.) and it starts to turn into a weird, hybrid engine instead of an iron block engine.
I have thought about porting my 3.5 heads to the same profile as the 5.6 heads. I will see how ambitious I get. I can always pull the heads later for work if I choose.

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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