I have also had this problem. On my 79 Vette I changed to a Holly carb and it starts now with two pumps no matter how long it sits. I also changed the carb on my 75 Grandville to a Holly and it starts on two pumps no matter how long it sits.
On my 69 bird I have a Edelbrock carb with an electrical pump. Once the carb is primed with the electric pump it starts with two pumps no matter how long it sits. I really like the Holly carbs. I recently purchased a 65 Riviera with a dual quad setup. It has Edelbrock carbs. I have to pump the piss out of it after it sits for a long time. I am going to try adding an electric pump to prime the carbs and keep the stock mechanical pump.
Just my .02.