Stalling Issue (Slant Six w/Holley 1920 Carburetor)

I'll probably tic off some experts, but I'll say that is likely normal with many carburetors, especially a simple one like the Holley 1920. When you floor it, the choke is forced open, so you no longer get the idle enrichment. The accelerator pump is set for a warm engine, so probably doesn't squirt enough fuel when cold. Just drive slow when warm, which is smart anyway to not damage the engine, like say spin a main bearing. Sounds like you are lucky. I went thru 4 Holley 1920's in my 1969 Dart over 20 years. It always idled lean and would stumble and even die off the line. No mechanic could figure it out and I tried solving "a vacuum leak". When still a problem with a new long-block, I figured it must be the carburetor, tried the 4th and it ran like a dream. 1920's have a sealed metering block which gets clogged and most rebuilders don't touch, so many bad ones on the shelf.