When I fitted the SRX8 brakes on my 3.0 car I just fitted the discs and carried on using the pads from the GT Pro brake kit which is now discontinued because the srx8 pads were too thick and didn’t fit properly.
On the 3.1 kit there are revised fittings. So to fit both the srx8 discs and pads You need the GT 3.1 plate mounting kit.
It’s brake plate mount set GT 3.1 part number 600988.
http://www.serpent.com/product/600988