Back at the Cabby!

For a variety of reasons, it's been quite some time since I posted on here.
I have made some slight progress on my '83 Cabby including getting it registered on Irish plates so now it's 83 WH 8003 which i'm happy with.

Work to date has consisted of fiting a replacement drivers side window regulator, timing belt and tensioner, spark plugs, leads and dizzy cap, oil and filter change and shimming the tappets. I've also fitted a new thermostat after flushing the cooling system and replacing the heater feed hose, fitted a NOS radaitor and a fan stat switch as well as a new coolant reservoir pressure cap.

The engine is running well if a little hesitant to start and the idle is erratic while warming but I'll eventually get around to that.
Right now, I'm having my ass kicked by the exhaust manifold, or the removal of it to be precice.
One of the manifold gaskets is blown resulting in an annoying noise particularly at idle and I'm attempting to replace it but the bottom centre nuts are a sod to remove. I'm talking about the two closest to the downpipe and due to the design, I'm unable to get any leverage on them wih anything other than a combination spanner which hasn't the torque to loosen or snap them.

A couple have sheared off where the nut sits against the flange but I recon a stud remover will cope with them when there's access.
Anyone have any successful solution to their removal as I'm trying hard to not have to remove the head?
It's realy a stalling point at the moment and should have been a simple job so I'm hoping for either a solution or a miracle.