And check all vacuum line ends, the accordion hose between the MAF (or AMM, of you state this already), air intake port (manifold, if fuel injected, we call these intake and exhaust ports, if we were in the Commonwealth) gasket (check air intake port nuts for torque, yet don't crush the gasket, specially if it's original, injector seals.

The leak in the header pipe can be a cause. Also, check all exhaust unions from the cylinder head to the catalytic converter intake.

The hardware is weak between the header output and the catalytic converter intake. You may have the spin metal donut gasket or the flat flange interface. Inspect it and remedy.

Be careful of old cylinder head to exhaust port retainer hardware. The studs can rust and futzing with them can cause the studs to fail.

Use water with some dish detergent and spray around exhaust unions for leaks when the car is cold, spray on just before and after you start it up.

Also, be darned certain the mechanical hangar from the North American passenger side bell housing to the end of the header pipe is in place and supporting the exhaust system weight at that point.

Good luck finding a junkyard header pipe to fit you 240. New ones are not so pricey.

Hope that helps.
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