I never cut any more than abosolutely neccessary! You can always cut more, but it's tougher to put some back! Do some measuring for tire/wheel clearance and then figure out if the frame rails need to move, and if so how much. Once you see how much they need to move you can measure from the inner existing edge and see what needs trimming. If the frame needs to be relocated in, make the cut in the trunk into the inside edge of the frame. That way the wheelwell wont be an issue for any reason.
If you want to leave your hinges just cut the down support loose from the inner wheelwell, and leave it hang while you tub. Once you've tubbed the wheelwell you can then make up metal to reattach the support for the hinges back to the new tub.
Sometimes it's easier to cut off the rear frame rails first. Then narrow them outside the car and build up your backhalf. Then you can move it under the car and jack it up to see what obstructions you have and address the wheelwells then. After cutting away the sheetmetal you can raise the frame into position, weld it in and do any gussetting. Then you can start rebuilding the new tubs where they need to be with the modified frame.