A Cummins will start without the heaters in 0* weather.....no extended crank....maybe a little rougher, but it will start.
Like said.....I think you have a lift pump going south, or a loose fitting or line letting air get in the fuel line and causing a drain back. I personally would go to a FASS pump/filter system if you're going to upgrade...not a fan of in-tank-pumps !!!
Get a pressure guage and watch it during the start sequence...it'll tell ya.