Not metric but the 1/4″ work fine. The head is a bit bigger than the originals but they work fine. I see them on quite a few guys karts (including mine) out here on the west coast.
You know Mark I think Chris is right. I remember last time I had mine off it had a thin shim between it and the hub. I just thought the prior owner had made something to adapt a different hub on. Maybe it was OEM Anderson afterall.
I’d think you will cutting it real close with a 3.5 litre tank. I’d go at lest 5 litre to be safe. Especially if you don’t pick up all of the gas in the tank.