I don't know of any manifold for the B21-23 that take two SU's, but there are lots for single SU and Stromberg. You should run the engine mounted mechanical pump rather than work with the FI pumps. The carbed engines all had breaker point distributors - if you want simple, use them. The earlier B21-23 electronic distributors can be stand alone, unlike the LH systems, so that's not a problem. Yours should have an advance unit on it, or perhaps a dual diaphragm with advance and retard.
I have done this a few times - my 80 242DL has a B23E with a single SU and breaker points now. Simple, and will never let me down.
The only strange thing I encountered doing this engine is that some of the intermediate shafts do not have a machined eccentric for the mechanical pump. The raw casting of the eccentric is there, but it is not machined smooth, and would eat the lever arm of the pump in no time. You'll see it through the hole left when you remove the warm-up regulator bracket from the block.