for the turn signal unit you have to mill (or grind) the ID until it fits. This makes the insert clamp piece pretty thin so you have to be careful. The screws that hold the halves together are total crap also, so we take 5mm stainless Alans and turn the head down on the lathe until they fit the grooves, and throw away the 2 screws they send with it.
These T/S units in general are a PITA.
Also, before you get too deep into it, test the functions. Often times I have to rebend the signal arm to allow full movement and I have to modify the terminals inside for better connection, and reshape the dimmer contact so it doesn't make contact when you don't want it to (many of them are so tight they will flash the brights when you put on your signals).
Post a picture of your hub adapter for the flat 4. I've used several over the years but not seen one that interferes with the horn button. Anyway, easiest solution for the flat 4 is the Grant GT 3 bolt hub adapter. Same bolt pattern Grant and Flat 4. You can find them all ver the net sub $50 (Summit Racing usually has them in stock)
EDIT: I don't think the 6 wire has the dimmer built in to the arm like the 8 wire does, so you may not have to worry about the dimmer ground contact tab, but you will have to add a momentary switch for your hi/lo dimmer if its not in the arm.