Yes I bet the sock on your pickup in the tank is dirty if you have not checked it.

I have a 440 and a 340 with stock mechanical fuel pumps and fresh socks in the tanks. They both start after sitting for months. Yes I have to crank them for 15-20 seconds while pumping the gas petal but they start consistently for more than 10 years now. The 440 has a 6 pak and the 340 has a edlebrock 600 on it with mechanical chokes that work. The chokes help too. I haven't had the air cleaners off in years literally.

Besides the sock check for any rust, even the smallest pin hole will cause problems starting in your steel suction line. Can't have any leaks in your suction line, check it.