I converted my '65 121 this way a few years ago. You'll need the caliper mounting flanges (consoles) out of a '69-'70 121/122/123, part no 673581 (left) and 673582 (right). Definitely not avaliable at the Volvo dealer today, though... Luckily, calipers (Girling) are identical in the 140, part no 684432/684433 left/right, and therefore should be easy to find.

The brake line/hose setup is completely different in dual circuit cars and the most time consuming part of the conversion. Finding a complete donor car will make this part of the work m-u-c-h easier. One really irritating detail: the four tiny consoles welded to the cross member, supporting the lines were they meet the hoses. Switching crossmembers or cutting those consoles out and welding in your own car, that's the question.

Also don't forget the pressure regulator valves in the rear circuits. There are a few different calibrations depending on body (sedan/wagon) and rear brake cylinders. One more reason to pick everything out of one car. I've also been told late rear brakes (1968-70?) were self adjusting and a bit less troublesome. Unfortunately I don't know more about this since I converted my car with disc brakes instead.

/Martin
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'65 121, '73 165