i see what you're up against .
well done with the magic marker trick .
the right side casting " finger" is higher .
( milled with the gasket face .? )
it never occurred to me that it could be in the way and impinge on the tappet casting
.
this upper pic , snagged from eBay ,
looks like someone's filed the right side
to make space for the tappet casting .
...
and another pic showing the "finger" lower
so less in the way

I found some marking at the barrels, 10/69 at the one I going to use and 51/65 at the reference. My guess, right or wrong that means week/ year of make. Conclusion following my guess is that the "smaller" house of the tappets on the older barrel should give no problem for that reason on any cases. The housing of tappets interfere with the bridge at the side towards the rear, not at the top. The bridge at the case I will use is flat to the gasket surface all the way out. Tried to post the Pictures according to Gavin´s instruction without success. Loong weekend. I will be back on Monday.

I have now grinded down the front side to the right of the tappet housing. At the corner it is about 1mm down and diminish to zero about 10mm in. The height of the grinding is about 15mm from bottom of the barrel. The barrel fits perfect now. The studs with collar was grinded and polished down from the approx. 9,50mm to about 9,45mm. Any opinion to if should get new ones or use the ones I grinded down?
Still problems with Pictures...

Presumably the cylinder fits without movement?
You should blue the tappets and cam, install the cylinder with the tappets and turn over the cam to see if the contact is centered on the lobes.
Perhaps more explanation of how you are trying to post pictures?

The ground collars should be OK, after fitting pistons and rings before finally tightening the barrels down , spin the motor over a few times , if there is no obvious binding ( there shouldnt be, unless something is seriously out of whack) , you are good for the final nip up. Daves suggestion for bluing is a good idea.

Yes, the cylinder doesn´t move. Thanks for your advises. I will do that.
Regarding the pictures I signed in for Flickr but then trying to log in it says that it can´t find my mail address..

Your E-mail address has to include the full extension ([email protected] or [email protected], ...)

Tried that but no success..