Not directly familiar with the F7 but the SR61 was fitted to the last of the F86 of which I have one. Range change is the usual wire control from gear stick to internal cylinder at rear of box and split was also wire control from engine hump (or dash on the 88 and between seats on N series)) to external air cylinder on right side of box . As it is pre-select there are interlocks with clutch etc. You will have an air line to the clutch servo and to top of gear box. Will have to have a look at splitter.
It was only with the later SR62 that splitter had switch on gear stick.
Was that Volvo i advertised the same as yours. It will be in the for sales. 8 wheeler . Check it out . If it is the same i may be able to have a look for you. I don't know a lot about the truck but think it has the original gear - box. Let me know if i can help. Dave