For a multitude of reasons, PCV is very important. Especially on those old cars, so i'd make sure it at least works. How it's done doesn't make all the difference. Some people use the SU carb's port to vent it off(later method), some use a pipe that runs down to the road, cut at an angle.(former method) Being cut at an angle, the pipe create a low pressure when air gets blown past it, venting the crankcase.
The former method makes gunk and oil deposits settle in your manifold and carbs, so the use of a catch can sure is recommended. I use an aftermarket one myself, too. Like many other cars, my car doesn't have a PCV valve, by the way.(76 Triumph TR7, also a leyland product with SU's) It vents the same at all times, and I never experience backfires or any other mixture related problems, either when disconnecting or using the PCV system. I don't think backfire issues normally occur when the PCV system is faulty, or when the valve is taken off or broken.
Theoretically, the PCV port on a SU only sees the vacuum present after the airfilter, so that's almost no vacuum. The valve in the PCV system is a minor drivability aiding feature, and removing or replacing it with a different one makes little difference, if indeed connected to the SU. If your PCV system is connected to the manifold though, it is a whole different story, and using a valve with a different spring rate may cause lean running.(and thus backfires)
