GameBent said:
"seems to me that that topclamp could be cut and welded to fit most bikes quite easy."
Says the guy who can actually weld.
Not all of us have that coveted skill.
True, but if i lived some where near you, I'd be happy to weld it for you.
seems to me it has a bolt on the top of the clamp(to clamp it!

)
consider to put a short rearfacing stem first and after that the clamp.
another;it might not be the the most atractive solution but maybe you could use a spacer,
that way you keep the clamp intact, and the fork can always be swaped to another bike.
I see a lot of guys here, building awesome bikes without welding, being able to "weld" isn't everything,
you gotta to have skills to make a frame geometry that actually works.
I tried twice started enthausiast, but in the end they didn't do it for me.
guess i have to learn, i wont start on another frame build before i have the space and possibility.
but thats another talk.