slant 6 stalling at higher speeds

That Holley carb likely needs a top to bottom rebuild. I took mine off and run something else, but if you rebuild it get a good kit that has the ethanol-resistant rubber parts.

Those 1920's are notorious for off-idle stumbles and sometimes not so off-idle. Mine would do it randomly, so after it finally started leaking I switched to a 1945 and couldn't be happier.