converting an 8 3/4 to the correct sure-grip

you will be fine if you have a factory "A" 8 3/4 rear. The only thing you will need to do is shorten the driveshaft and have it reballanced. You may need to transfer emergency brake cables and get a new brake flexline. Are you keeping SBP axles? Do you have 10 inch brakes? Either suregrip is much better than an open rear. Bolt it in first then add a suregrip later if you want.