The intent of my last post was mainly to provide information. I'm not "fer" or "agin" the UN2.
I'm just shaking the apple tree hoping to get a ripe apple to fall.

Wouldn't the Carter YF 4371S be a better "universal carburetor", to use on a 261, than the UN2?
Disregarding issues that may require modification to the throttle linkage, vacuum advance and choke etc.
Listed application for 230, 250 and 292 engines.
Throttle bore 1 11/16"
Main venturi 1 5/16"
Metering jet 0.104"
Metering rod (3 step) 0.078", 0.052", 0.045"

If yes, why not clone the Carter YF instead of the Zenith 28/228? Difficulty in manufacturing? Patent issues?

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."