Dan,
....just for clarification, you don't modify anything on the car and it's totally reversible on Bill's kit also. You do need a donor TR3/6 bell housing though, you could pick uo a non op box cheap and use that and still have your own tranny for posterity.
Herman makes a fantastic kit and was forward thinking in his application. Bill makes a great kit too and they both perform the same function of allowing a 5th gear.
Hermans is very factory looking and Bills looks nice but not as sanitary per se.
The big dif for me, and the reason Bill made the kit was the difference of $1200.
BTW, the other reason I like it was it's very cool when someone gets an idea (yea, I can make that)and you make it work. If you look at a lot of the stuff Kastner did when modifying TR's it was that same kind of 'cobbled look' that went like stink and everybody copied.