RHS is rectangular hollow section and will be 3mm thick.In my opinion,no you do not need to start again,as you have probably started on the lower frame which needs to be strong anyway.Keep it equal both sides and fill in the rest with 1.6 or 1.8 .or 2.0 mm ,whatever you have over there.If you plan on putting a powerful engine in (150 hp or more),then it will be just fine as I hold the view that the chassis needs a bit more strength for the higher power outputs used.
There will be a very minor weight increase for considerable extra strength and it will be easier to weld,especially if using an arc welder.
Over here a very popular clubman 7 is called a Chevron,had one myself and the main framework was and still is 40x40 x3 RHS,I really liked that chassis design as it was very strong and resisted rusting and was easy to weld brackets to.
So no, use what you have and it will be just fine.David
