It depends on the size of the piston and what type of calipers are/were on the 85. Also the proportioning valve may have to be changed out w/ an adjustable unit. If the 67 is a dual circuit master cylinder (it should be) brake line re-routing should be minimal. It can be done but there may be a lot of work involved. What is wrong w/ the 67 unit? If it is a matter of the cost of a new 67 unit then look at a rebuild kit. You will need a brake cylinder hone which isn't that expensive.