I plan to mini-tub my 67 cougar using a repro 67-8 cougar wheel tub. I’ll cut out my existing wheel tub leaving approximately 2" behind, then butt weld the new one to the existing tub (obviously cutting the vertical flange off on the new one first). I like this approach because it only leaves behind one weld seam. The other way it can be done is to split the existing and put a filler piece in, but that means an extra weld seam to grind down.