After I spent some time laying out where to cut, I had to think about how I would cut the hole in the tank since its been holding gasoline for 47 years.
I've read a few ways to make the tank safe for cutting, like filing with water or taking to a radiator shop to flush, etc. So I will tell you what I did, however I'm not endorsing it, but it worked for me.
I had drained all the fuel out last weekend. Since I knew I would need to flush out metal shaving with WD40 after the cutting I thought, why not just flush the tank before with WD, way less flammable and WD cuts gasoline.
So that's what I did. Gave it a couple good flushes, then cut the 4 1/2" opening with a jig saw. I kept the blade soaked and cool with WD while I was cutting for good measure.
After the cutting I wet shop grade paper towels with WD and cleaned the inside of the tank best I could, then poured in about 10 ounces of WD and swished it around and ran a magnet through the WD and collected more metal shavings. After draining out the WD and wiping it up good I decided to leave the magnet in the bottom of the tank.