Need some wheel tire sizing help

[The_BullDog]

Ok so its time to folk up the bills and get some rims. I got a 77 C10 long bed with stockers on it now. Looking at centerlines legend retros. 20X15 in the back and 20X8 or 18X8 for the fronts. I have 3in drop spindles in the front with 1 coil cut. and i got a 5in flip in the rear. I am looking for someone that has experience in what has to be done to get this to work. I am guessing shortining the rear end is in my future cause i want that huge lip. but.. how much should i get cut off? am i going to have to toss my inner wheel fenders in the front to run the 20's or 18's? and am i going to be able to do this without messing with my frame? any idea on what size tires i can run? any help will be great!! help me get this project rollin!