I have a pair of decent Willans 3inch, 4 point harnesses, but on the passenger one - even at the rights shoulder setting, its still too lose for a lot of my passengers.

Silly question I suppose, as I 'think' they're stitched, but can they be shortened further somehow? or do I have to buy another, and if so - do they sell them in different lengths?

Could you make the pax seat move forward, thus taking up the slack

The stitching is an integral part of the safety.
You would have to get them done by a seatbelt manufacturer or a company working with aircraft strops
If the stitching is not done with the right thread in the right way you don't have a seatbelt anymore.

Just a side note..
i think someone said seatbelts should be changed after an accident
This is because they 'stretch' - only once - on impact and absorb..

maybe your passengers need a booster cushion - you could make some race style foam ones...

If you *have* to get them adjusted then a parachute rigger might be able to do them for you, but otherwise I would buy new,.. and always new. Afterall it is your passengers safety at risk.

Loose is almost as bad as not wearing one, your head can easily whack into the roll bar, the same affect as being hit with a scaffold pole!