You need to do more homework on this project and have a design plan - preferably in a CAD system so you can more easily calculate volume and weight.
You also need to understand strength of materials far better. Are you using grey cast iron or a nodular iron for instance.
They ran locomotive shops and many shipyards with 8 and 9" vises, I've seen pics of a German floorstander that had 12" jaws. I don't know the particular use, probably shipping
Two guys here teamed up and built a massive 13" floor stander up in Alaska, out of plate steel mostly. IIR it was 1600 lbs.

Another guy made these (he's in the pic)