D1 3.9 v8 air leak

Hello all I think I have tracked down part of my running fault.
I have found the rear of the inlet manifold on the offside to have an air leak. I have checked / replaced all the vac and breather pipes etc.
I have found that if sprayed with carb cleaner in this back corner the idle lowers and almost creates a misfire.

I suspect the injector seals are possibly cracked I have brought a set to replace, my question is do the valley gaskets have a known problem with air leaks?
is this something I should replace while iv got it partially stripped?
Also is there anything else In that area that could be causing an air leak?

Thanks for any help

what about manifold-plenum base or plenum base-top

I'm tempted to take it all off I think and change/ seal all the gasket.

How bad of a job is it to do?

Is there anything troublesome or tricky?

to get to the inlet mani gasket, the plenum comes off and the manifold. thats it basically