Normally you should only need to balance one carb body against the other and then balance shouldn't change regardless of jetting changes.
So to me it is all a jetting issue (guess you already knew that tho!). Backfiring or spitting under accel usually indicates leanness, a rich mix will normally feel "heavy" as it revs.
Just throwing it out there but are the throttle plates around the right way? Some have a groove on the edge that should pass across the progression holes as they crack open, I have seen them inserted backwards before..

EDIT: Wurzel has my carb book which gives basic starting points for weber/dellorto carbs but I can't see them being that different for a Solex, I mean you are still trying to put the same amount of fuel in per cylinder regardless of carb type? Anyway I don't have it on me so can't tell you what it would be for your motor sorry, maybe contact Wurzel?