Fusionking maybe you could learn the metric system?

T45 is AWESOME material has excellent properties, like Fusionking says though Aluminium is a bit different it does not have the inherent strength of steel. Typicaly a aluminium section of a motorcycle frame will be roughly twice as thick as a steel section. Aluminium will distort a lot during welding and you will have to weld it on the jig and be mindful of your cooling rates.

I seriously suggest you buy John Bradley's book 'The Racing Motorcycle Vol 2 , A technical Guide for Constructors' John is a highly respected engineer(and lecturer) and is always willing to give his advice .