Rig a new fuel source, a temporary fuel tank that is above the highest point in the fuel system, so that gravity will overcome any deficiencies in the lift pump or any air leaks in the system. If the engine will start and run, then you know you've overcome the fuel priming problem. However, you would still need to resolve the root cause.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC