It's not necessary to heat the carbs per se. What you need is heated air. You can do that either to the air as it enters the carbs, or after. The late integrated iron manifold is best for smooth running in cold weather - in Canada it gets cold a lot....
The pre-heated air is also a good idea, although the manifold heat should be enough.
A viscous drive fan is a great idea too, and blocking some of the air into the rad is as well.
A good block heater, and 10W-30 or even 5W-30 synthetic oil will make the engine perform like a summer day. I run 5W-30 in my B23A year round, no air pre-heat, with the stock water heated intake manifold (and a 188 thermostat) that the OHC engines use. No problems. With the integrated iron manifold you'll have the best luck. The aluminum manifold is not the way to go.
It's a Volvo after all - from a cold land...