opening rear wheelwells for big tires

we are moving the springs in and mimitubbing a 68 barracuda.i would like if possible to fit 33x18.5 tires in the rear even if they stuck out a bit.ive seen pics of cars back in the day with them opened up but was wondering if there was a tutorial of how its done.my lower front of the wheelwell has bondo and i have patch panels so ill be working in that area anyway.