Certain points to be noted with the head diagram that I got from Keith are -

1. The silicon undoubtedly will swell in contact with the fluid that it meant to seal against, so the problem is....? (There isn't one as its a benefit.)
2. It was an experimental head and although it worked well in an engine that had the boost port blocked off, fourth vestige of a port under the exhaust blocked off, a small fixed venturi designed for good fuel draw (not power), used wide blade props, and had the low timed Akrobat liner in it, it was still a work in progress and as such the length of the silicon tube could be tailored to suit unlike O rings that need far more length when doubling up for extra seal,
3. The original maker was the MVVS distributor at the time of making and the sizes in the diagrams suited the button heads 49's of old and as such may not have the ultimate dimensions all engines, and being a distributor had access to parts for tinkering that we can only dream of.

Anyway, probably best to talk to the source if you want to go further with this design.