Yes the R1's are 40mm but with far less restriction than the SU's and 1 carb per cylinder. They are being used extensively in the UK on a wide range of modified cars. The only downside of them is that the spacing of the carbs is too narrow for the B20 head which would require a bit of fiddling to get them to fit properly. A manifold would have to be made up anyway so this is no problem. The only other thing is to get the the jets drilled to the right size for the engine. I gather the jets are the same thread as for Webers....
The other alternative is using the D-jet manifold with ports welded in, as the modified head I have is pre-FI, and then using Megasquirt.
I tried a pair of 45 DCOE's recently but while I could get the car running reasonable at idle up top they were way out. It was suggested they may have been 2 Mini carbs added to a Volvo manifold.
All I am trying to do for the moment is get the engine running well on the SU's and then decide where to go to next.

The head gasket came from WA several years ago.