Hi Neil;
You're right in that any dual carb installation with a cross over tube will be less "fussy" on the balance aspect. And of course all Volvo manifold had the cross over. The difference in the secondary throttle plate manifold is that at idle the front carb feeds the rear cylinders and vice-versa, which allows greater heat to be imparted to the mixture and a leaner mixture is then possible. Webers and the like that feed one cylinder per barrel with no interconnection between the runners are very fussy on balancing.
Putting flow meter on the carbs for set-up is worthwhile, especially for the off-idle throttle response as load is applied. As even the Strombergs have an idle stop screw one can assume that at sometime in the past somebody messed with them, and putting it right is easy.
Autobooks - I didn't think anyone used those manuals anymore, and Chiltons.....well. It's the green Volvo books for me.
Rhys