It was somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "BigWallop"
saying something like:

Are you sure that the soil stack is only 90 mm (3.5'') in diameter Steve, as
anything less than 110 mm (4'') is not allowed for solid waste soil pipes.

Old CI soil pipes are sometimes to be found in 3.5". There's an adaptor
available for them to allow pan connection, but usually only in straight
connection off the shelf. Multikwik or McAlpine may have something
suitable for the OP, but it would mean some catalogue searching.