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Steve, you can buy extension leads, usually around the 1 meter mark, and run them back so that balast and igniter can be located under the seat/etc.
Or if that isn't preferable, you could consider an option that I ended up with at the time, that was to fit a leather tool bag to the front end and locate the balast and igniter within that,
(in my case to the risers above the headlight).
The two other things to consider is whether you buy a conversion kit that is "plug & play" or the kits that require a battery+ and earth connection, and to ensure that you choose the H4 Hi/Lo version, not the straight H4 version.
There was one other thing that I thought about when trying the HID's, and that was about the recomendation by some maufacturers that HID's are only turned on after the engine had been started, obviously regarding voltage drop when cranking the engine.
I never modified the headlight circuit so that I could isolate it when starting the donk and left it as per the Harley set up with the headlight always on deal, but probably didn't run the HID's for long enough to find out if it was going to be an issue or not.
Hilly has done a lot of research and has experience with HID conversions, so he might have some comment about that.
Yep, a larger headlight is another option if yar happy with the looks, gives ya the chance to choose a good quality reflector/lens as well.
From what I've seen ya gotta find a real long 5 3/4 shell or go to 7 inch.
It'll be interesting to see what advice the reccomended installer comes up with, keep us posted on how you get on.