Wow- that's a tough one. The 160 PSI cranking pressure on all cylinders rules out a number of possibilities. I guess if it were mine, I'd try to run it with a points-and-coil distributor, just to rule out any weird HEI situation. Any possibility of an unnoticed intake-to-exhaust passage- like a casting flaw (hole) in a manifold's heat-riser passages? Or wrong intake gaskets allowing such an intake/exhaust leakage? I'm sure you've already checked the sparkplug wiring to match the firing order at least a half dozen times- and the cylinder numbering convention of a 403.
Aha! You got the battery hooked up backwards, so it cranks and runs (??) backwards!
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