First, are you ever planning to run in any type of class, the mini tubs many effect what class you can run, even at local events. Look at the future you never know what it may bring...
Second, if you plan out the chassis the cage design will never interfere with the mini or full tubs. The exact length of the axle will depend on the rear, rim, tire etc... I also suggest 9" bearing ends, and a good aftermarket disc set-up.
The links that Hllon4whls are good
Chromemoly is not quite half the weight, but it is lighter and allows you to use thinner tubing (both in diameter and thickness). If you ever plan to go 8.50 you'll need to do a 25.5 chassis, SFI approved.