It has to be done as a package with the main body. A lot of work.
When I was looking at the late 69-70 predecessors to the 4668, the transfer slot circuit was seperated from the idle circuit.
Transfer slot got its own air bleed in the main body and IIRC fixed restriction.
The idle circuit has an adjustable but hidden restriction after the air bleed. The air bleed in those cases was externally adjustable.
IMO its best to use that carb for its original use, with relatively minor mods if there's no reason/desire to keep the emissions characteristics. I'm not suggesting emissions control is always bad, but some of those early setups were tradeoffs for fuel economy and even sometimes NOx in favor less HC and CO. I wouldn't blame anyone who wanted to run a richer stronger idle at lower rpm and a more normal initial timing to go with it.

I don't have a '71 service manual, but that's probably the one with the details of this carb's secrets