I didn't see it mentioned in the other posts, but it sounds like you might still have the B18 motor in there. Is the intake manifold and exhaust manifold all one cast iron piece, or are the two separate, with the intake being cast aluminum and the other iron?

To get the manifold free of the downpipe I usually remove it from the head too, granting more room to wrench with. More of a pain if it's all one big iron thing.

The gasket is still available new from several sources.

If when putting it back on you don't follow up and keep tightening those nuts after running it a couple of sessions, you'll end up with the same leak again. You'll probably find when you get it off that the studs are worn down on the collar, from the downpipe vibrating and slowly ebbing away at the stud.

I would also take the opportunity to source an early B18 intake & late B20 exhaust manifold before putting it back on, but that's just me.

Cheers, -Sean
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'66 122s, '70 142s, '74 142e... Blue is beautiful.