No prob
I wil also do a 6870 build with R785/RS805 brakes and DT Swiss R24 Spline db wheels.
Should be finished in about 3-4 weeks.

Martink, check with Bclem here http://chinertown.com/index.php/topic,809.0.html as he also got a 54 like you, and from his pictures there may be more room between the tire and the seat tube on the 54 than my 58.

Thank you! That's very interesting... I will happily report how the spike tires will fit, when everything has arrived. Thanks for your picture!

Thank you very much for your pics and your quick reply.
I think the small vertical distance here is too close in my opinion.
A few more millimeters clearance there would make the frame perfect.

I will order the Xerxes tires and at least try it out for myself.
Chances are good. The spikes are mounted on the side of the tire:
http://www.bike24.de/i/p/5/6/117365_02_d.jpg

But your tubeless Pro One's are probably a bit deeper.

The side clearance should be more than enough tough.