My 64 is a little cold blooded for the first half mile or so, then it runs perfectly. (I am so glad I have a Carter BBS.)
Seems a lot of people struggle with the 1920.
As mentioned already, choke operation and heat stove need to be right. Do not bypass the heat stove!
If your choke operation is bad,
@slantsixdan swears by the electric choke conversion. That will solve that problem.
"Timing set properly" Try 8 to 10 degrees initial timing with vac advance hose plugged.
Use the extended NGK ZFR5N #3459 plugs without washers. That puts the spark deeper into the chamber and will help burn your rich mixture.
Long block rebuild only? A good solid bottom end will only emphasize shortcomings on the top end. Is the head in good shape? Do a compression check to verify all six holes are close to equal. You can rent a compression gauge from a big chain store for free. If the head needs to be pulled for a valve job, have it milled .080 or .100. You lose compression using a modern composite head gasket. As Charlie says, there is no downside to milling the head. (Unless you don't want better performance and gas mileage)