Talk to Rimmers about it. I'm pretty sure that when they did list brake parts on their 260/R75V8 pages (they no longer list any specific 260/75V8 brake parts !), the part shown was the same as the ZT 190 or R75V6 (2.5) /Tourers, namely SJC000080. They still list that part, but only on their brake pages for 'MG ZT' and 'Rover 75', where they state it to be suitable for "all models". Probably a good idea to ask though, as I say.
https://rimmerbros.com/Item--i-SJC000080
David