Fuel/Water tank gauges

I want to put a good digital tank gauge set on all my tanks in my 1978 53MY. The Snake River "Accu8" looks good, but I don't know how to get the old gauge plugs out of my tanks. All diesel and water tanks have the same gauge plug which is a very large (about 2.5 in) bronze hex cap with a dished depression in the top which I believe used to contain a sensor of some type. The cap looks like a big hex nut and has another same size hex fitting under it. Almost impossible to get a wrench on the lower piece to hold it while a put a big wrench on the top piece. I tried WD40 and a huge pipe wrench on the top hex cap, but it wouldn't move and I'm afraid if I put any more torque on it that I'll break the lower fitting out of the fiberglass.

Any ideas?

Not familiar with the particular item but a technique to loosen stuck/siezed nuts/bolts that is very effective is to first try to tighten - that's right, TIGHTEN the bolt/nut. Once it moves in the "tighten" direction (normally clockwise) the tiniest bit, it can be loosened with no problem. This really does work most of the time.

I believe it works because the "we're-not-going-to-let-you-loosen-this" gremlins get confused when you try to tighten it. They then rush to change their grip on the object to counteract this. But now they're out of position and taken by surprise...

It is possible that there is a more scientific explanation for this phenomenon.