Biggest slick I can run?

m/t 28x10.50x15 will fit real good i don't know if the 10.50w will have enough clearence

i don't have a frame notch but i am going to try and put some 10.5w's on mine. i have some 15x10 weld super lites with 4 3/8" back space as best i can tell with a tire mounted on the rim. i mounted one on my car and with a 275/50 radial tire i am about 3/4" away from the frame so it's going to be tight. i am rolling my fender lips tonight. i think the section width on a 10.5w is 13.1 or 2. i would think with your frame notch and that back spacing an 11.5 slick should make it.............Bob
Just air up the bags because the tires will be under the lips not inside them. You need more backspace to fit either tire properly. That is the same wheel I run & get away with it because of gnx flares.
What is going to limit you is the BS of the rim. If they were the 5.5 versions or even a different brand with a 5in BS, you would be better off.

If the rear of the car is high , with air bags, MT 28x10.5 would probably work.. At that point there is little to no reason for the notch.

You could stretch out a set of 275/60 MTdrag radials and do some heavy work to the fender lips and probably get away with it at stock height.
i have the 10.5s on mine yes hes right about the air bags might have to lift the car slightly in the rear but fits with no problem