It did have flat top twin carbs on it but they were knackered so we've started with a twin 1 1/2" SU conversion (really similar carbs).

Both butterflies are closing the same amount. The rear carb has it's piston way higher than the front one and is definitely flowing more air.

I intend to keep the pcv valve if I can find a replacement.

I wonder if there could be a leak between inlet and exhaust somewhere on the manifolds.