TD, there is one other option, install the European 30 tooth countershaft pulley, I believe you have to have the ecm calibrated for it though.

 

Just a curiosity, if your trying to put 6 gears in a space designed for 5, wouldn't you have to reduce the size of the gears? and would that affect the integrity of the gears having to make them smaller?

 

Somewhat related to the OP's question, in the 90's I had a 1982 CB900C similar to this one, only blue.



It had a 5 speed trans, and then a high/low range jackshaft between trans and driveshaft. Low range was a rocketship around the city, then on open roads I could kick it into high and it was like overdrive. Triple discs and air suspension front and rear.

Essentially had 10 gears. Normal riding in high range still accelerated good and could easily cruise 100mph+ all day long. Low range, you better be hangin' on up to about 80mph, cause it had like an 8500rpm redline and about 80hp. Fun ride, but still not a Harley.

H-D should develop something similar to the high/low range thing, at least for the Sportsters.