i have no idea why you would want to do this...but they dont exist. Injection perfection dont do them as their quad throttles bolt straight on. You would have to get the old maifold and get a twin webber naifold of any sort and tig it up yourself.
I love webers but you are really going backwards.
Yo will also have issues with ignition if u dont use the ecu and ignitors you are going to have to do something there too.
I have seen a CA18 de-injected with twin dcoe's on it in a 1200 coupe and it was bloody slow. Sometimes "simification" is complication. If you are going to do anything go for a quad setup with a nice ECU. I gained 30% power just over the standard injection with adjustable cam gears.