Have you got a straight edge you can test the flatness of that flange with, and the flange on the manifold? Nuts need to be tight, but not monkey tight, or the aluminium can distort. ( but you know that)
Although not 180 degrees out, you can get it wrong so it fires on no.3 when you want it to fire on no.1, but I think you’re capable of overcoming that problem if it arises !
Could also be the stop solenoid failed, cam lobes in the hp pump do wear out so it doesn’t pump at high enough pressures to run ( more so running on red, that doesn’t have so many additives)
If you are smelling petrol, there is either a leak or trouble in the float chamber, have you remembered to put a little oil in the damper if it’s an su carburettor?
If the body is earthed properly to the chassis and battery, I suspect there is a connection that has been crimped onto insulation rather than bare wire in your circuit