Best bet is to talk to whoever will inspect and certify the boiler before you cut or buy any metal.
You don't see many Smithie type boilers that big. As the diameter of the main barrel goes up the ration between heating surface and water volume goes down. May have a job making 90psi and any usable volume of steam.
Top of my head I would say 3/16" stays, 1" spacing and 4mm endplates. But go by the inspector and/or do some calculations.
Edited By JasonB on 27/05/2018 07:53:15