There is no return line with SU carbs. They have floats and valves to limit fuel flow. Of course, if pressure is too high it'll overcome that the tiny valves are capable of holding back and will start pouring fuel into the engine.

That dial is likely a fuel pressure gauge but may also be some sort of regulator. Without pictures it's difficult to give a definitive answer.

I assume because there might be some sort of regulator, the original fuel pump is still being used? The first thing I'd do is disable it, bypass it and run a block-mounted mechanical pump. That'll keep your pressures to a reasonable level that should be manageable internally by the carbs...

Dale