benz, i ran into a situation on a 603 that may be some of your problem,i earlier gave up on this thread because of the bs and i am 64 years old and dont have time to go back and read all that has been discussed.
but i do remember you saying you had parked on a grade for an extended time, so there is a possibility that the check valve that is installed in the lift pump(fuel)has gone bad and letting fuel leak back to your fuel tank.
i was reassembling an older 615 engine and noticed that with the ip#1 port open for ip timming that i had a loss of fuel,i pump it up and fill the ip with fuel,dont touch nothing and down it woulg go,so the chevk valve wasnt holding and the fuel was getting sucked back to the tank. first one i seen to do that,but it could be happening to you.
if it were me i would start with a fresh battery,tighten everything up,crank till it starts or 45 seconds pass,clamp the rubber fuel line with a clamp(tank to primary fuel filter)let her set till my starter cooled off and repeat this process,would put a charger on my battery and sorta help it alone.
many ways to prove the check valve is bad but i am lazy and look for the easy way.
72 old cars