As one familiar with the 2 prong systems, I have an old 60's amp that I still play, but "someday" I'll convert it to a grounded plug!
The drill is, hold guitar and brush back of hand across mic, get a tingle, reverse the plug!
The condition is rough, and if it doesn't work?
That old of an amp really needs maintenance, and I would be hesitant to power it up without someone checking it out first. It may or not need tubes, but P/S caps for sure!
Reverb price guide:
https://reverb.com/p/gibson-ga-5-skylark
I built my 5E3 initially using a JBL D-120, it sounded glorious, overdrive as a 5E3 will do, but it was really heavy! I installed a Weber 12A125A which I think sounds almost as good, but is a lot lighter!
Peter, if it sounds good don't change it! You need to worry about having too little reserve, not too much. I can attest that having an 18W Marshall plugged into a Marshall 4-12 cab with Celestions is a really glorious sound!
I just wonder, WHY?
I bought a speaker cab from a fellow a few years ago that had staples holding the tolex inside the cab. After a year, I noticed wrinkles on the top and sides, I'll get around to recovering it someday, (but I'll use adhesive!)