Before throwing good money after bad, now would be the ideal time to rethink your carburation strategy. VW put twin carbs on all type-4 engines for good reason - they work better. It's your bus but I would be looking to sell that carb, not go down the rabbit hole of trying to make it work when there are so many better, more simple options.
Most folk sell them as a set - carb and manifold . Might be worth looking on VZI if nothing turns up on here.
Chris, I know you've got the carb already but it really is one of the worst setups for a type 4. I might be able to help with an alternative if you want to give me a bell. (Or pm me)