Around here (Northern Virginia) you don't usually buy the inserts and do the work yourself. Unless you have access to some serious machine tools, it is not a DIY job. You take the head to a competent machine shop, (one that has been around for a while, and is familiar with the process) and have them do it. Usually runs $150-200, plus new valves if needed. Turnaround time is usually a few days.

I am assuming you want to replace the horrible one piece cast iron 2 carb manifold with the extra throttles in it. (I had a 68 144, my first Volvo, many years ago) The manifolds you want are found on a 67 122 (in the USA, maybe on later years in other markets) a cast iron double-downpipe exhaust and an aluminum twin SU intake. If you actually have a single-carb B20A, and want to keep it that way, I'm not familiar with the manifolds on those. Or are you running a single Weber (they don't work well with an A cam, by the way)

Some of the later one piece cast iron manifolds can have the intake part cut away, leaving a usable exhaust manifold with some ugly lumps on it.