Hi Paul, i would suggest that you either get GBS to modify the manifold or you get a friend to mod it for you to start with and then see how the carbs perform, if it was me i would do it myself as then it would be as i wanted it, whether it worked or not in the end, as it has cost you far too much already in my opinion.
I was thinking of taking the 4 carbs of my old Kwaker but dont think i'll bother now, even thought of going old skool and fitting 4 Mono block Amals or even 4 -1 1/8" SU's but its all room for thought as i will just stay with my DCOE 40s as they are doing a good job at the moe.
Mike