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?