Jezusjones,
I'm doing a shifter kit installation on my bike which also has a very close tire to seat tube distance. I have mocked it up and I believe the installation is possible (I'll make it work on my bike).
Keep an eye on the Rotten Orange build in the photo section for more info on how I make this work. I have a number of similar obstacles that I have to overcome with my build, but it will work in the end, just clearances will be extremely tight.
Judging by the mock up on my bike, I may even have to round off the edges of the shift kit bearing plates to clear the tire... that's how close mine is. So, dont give up. I believe its just a matter of shifting the engine forwards as needed and shimming here and there until the jackshaft and related components clear both the seatpost and the tire.