I can use any head gasket, but it must be a B18 head, and the maximum bore is only 85.4mm.

I've put a ported B20B head on to a B18B in the past & it worked extraordinarily well. It's well worth the effort to use a 44mm valve & widen the chamber to the length of the B20 head gasket.

I'm not sure the ports could, or should, be enlarged enough for a larger valve than that to make sense.

There's plenty of meat around the valve to fit it + the outside 1/3 of the port (& the legs of the manifold) will flow more air than is possible of any valve you can fit. Larger valves increase low lift flow, which may not increase peak power, but does let the engine produce more after the peak has been reached. Larger valves do hurt torque at low revs, but that's no drama for a race engine.

Another tip for 2x SU's manifolds. Make as much volume between the base of the SU & the beginning of the runners. I'd use 2" SU spacers behind the 1&3/4" SU's & start grinding.