Cheers murtle.
I have an efi manifold. I will see how I get on with the SUs first.

It would also help if you could some how use engine vacuum to lift completely out of the way the SU needle and plunger, then only the butterfly of the SU would be in the air flow, just like an EFI Manifold. It may be possible to drill and tap the SU upper housing and attach via a solenoid an engine vacuum feed directly back to the inlet manifold.

Then you could switch between petrol and LPG but have the best of both worlds.

I was just talking about doing that with jrv8 a couple of weeks ago. It stops the piston in the carb from wearing due to lack of lubrication also.
I think I'm going to do this mod. There is a redundant vacuum take off at the back of the manifold.