[Arran]

I can't remember whether there was a table version of a 40-180 or not, I think so, but even if not the chassis will bolt into a slant front table cabinet without issue. With regard to adding a 6C4 under the chassis, I don't think I would modify one to that extent, even if there was room underneath. There was a higher end model using a similar chassis that did include a phase inverter tube I think, I would have to check the manual to find out which one.
On the other hand the 40-190 uses an extra double diode tube as a second detector, which is a waste of a tube socket, one could dump the double diode, use a triode-double diode, and then use the detector tube socket for a phase inverter tube. I don't know how much you would gain in power output though as the B+ voltage is fairly low for an AC set, you would gain some but it would also improve the fidelity.
What they did with the 40-190 was similar to the old trick some companies had of using a 6H6 and a 6J5 to bloat the tube count when they could have used a 6Q7/6R7. In most cases the 6H6 had the cathodes tied together and at the same potential as the cathode in the first audio triode. Some even had both diodes in the 6H6 tied together, or had the plate and cathode tied together on one side. In other words the 6H6 was a useless tube in that application.
Regards
Arran