It's worth checking, though 850 rear calipers should bolt on. The brake fluid pipe needs to bend a little though the rest of it is a direct bolt on. Same pads, bolts, rotors etc. I have done it on a 240 and it is a no brainer.
Berry Motor Group or Monash Prestige will have some, or there is a Volvo workshop in Moss Vale, not sure of the name, my father frequents it and likes them. They be able to help.
For parts try the above workshops, fcpeuro.com or ipdusa.com.
Personally an 850 caliper is 10-20 years younger than the 740 and bolts right on, that has been my preference in the past.
