New project

I have been trying to get a project going for two years now but
have been delayed by back surgeries. i want to build a rectangle shaped frame
bike. My question is what gauge or thickness is good for a beefy frame
i am going to put 26x4 tires on it so i will need a good front end to match.
motor size is probably going to be big maybe even a predator motor set up
any help will be appreciated.

thanks

Wayne