180 does look better but 160mm was the standard supported size for regular QR drop outs. Something about the leverage of a larger disc adding to the risk of the wheel popping out of the drop outs... but that was probably written for liability reasons. Obviously QR20 does not have that problem.
Old thread, but for the ones surfing the net it might be helpful.
This Disc Adapter Formular (FS) to IS Mount is for older Marzocchi's like Z1 & Z2 from 1996+.
I solved this problem now by doing this well known adapter as I have a box full with orange Marzocchi Z1 from the beginning and people contantly asking for it 2-3 times in a month. Not that mass seller product, but I like to fix problems.
I needed some years to get a master sample, but here it is : Made out of Aluminium 7075, HQ CNC maschined, anodized. May not fit perfect to all Disc mounts, but it is a comfortable base to make your Z1 running on disc just by little changes!