i think you can pretty much do whatever you want -- head cabs are really pretty simple.
since your chassis is "tubes up", i assume you mean that they're at a right angle to the control panel like the transformers. that means you're going to be doing something more marshall style.
here's a link to weber head cabs (https://taweber.powweb.com/store/wccabs.htm#head
) -- they include the dimensions for the marshall cabs. for weber, the cabs for the fender amps are the same, they just don't use the piping.
basically, you'll see that most head cabs are either 20" or 26" wide, 9" tall, and 9" deep. the fronts and the backs should be cut 1/16" smaller in both directions to account for the layers of tolex. the control panel cutouts should be sized to work with your chassis.
good luck! looking forward to pics as you progress.