Hello saltiqa,
I know I shouldn't be saying things like this but why not get one from The Healey Factory then take it into the nearest "supercheap" and see if you can match it.
I'd say that $14.20 + GST is worth it if you need one.
I'm not surprised that people don't know what length the belt is, maybe they're not that interested in that sort of thing. Yes there are people like that.
No I don't know the length of mine either but I could soon find the part no pretty soon if I needed to.
Do some of those people know if they have one you could borrow to get one for yourself?.
Best regards,
bundyrum.