How to narrow the third member in my 75 GMC

[streetmaster42]

I was thinkin about narrowing the rear housing so i can fit wider tires to look like a pro streeter. I've got P275/60/15's on it now, and they are not even wide enough for my taste. It looks as if i can fit another P275/60/15 between the set i have on there now and the frame. So if i do some fab work on the bed to widen out the wheel wells, i could fit some pretty massive meats up under there. When i starded the ground up resto a few yrs ago on the frame, i took out all the rivets and removed the spring brackets to clean them up between them and the frame, and bolted them back on. So there would be no problem with removing them again. Im not happy with the drop i got after i installed the shackle and hanger drop kit, because i still have atleast two more inchess i want to drop it. If i go and do a flip with the axle above the spring, the truck will be to low in the back. So now im intrested in the prostreet look, but dont really want to put out a grand right now, to slap a 4-link set up in my project, when norrowing the rear end is gonna cost some pennies. So i guess my big ? is, has anybody ever tried putting the spring hangers on the inner side of the frame so the lief springs will be out of the way when i put the wider tires on. I can put a drop lief in there to get the 2" more im after. The trucks not gonna be used for hauling anything because its just my toy. I do want to do a 4-link in the future, but not right now.