In '72 two friends of mine ran a BT29 in SCCA FB(1600 twin cam) in the slim sides so I don't think fuel was an issue. So, it seem the deformable structure rule came into effect in F3 in '71, at least in the area of the fuel tank which would explain the buldge in the fuel tank area of the Lotus 59/69.
Anyone have a FIA rule book from 1971
