Which rear 8.25 or 8.75 swap for new project

I acquired a Duster last month and it has an 8.25 rear in it already w/LBP. I will be running an aluminum headed 318 Magnum that made little over 400 Hp on the Dyno and will run a727 w/ a Transgo kit in it. I plan on 3.91 gears and already have a set for the 8.25. I'll need a suregrip unit. I have an opportunity to get a 68 B body 8.75 rear complete for 300.00, but I will still need gears, SG etc to convert it. Either rear I use, I will need to move the pearches since I have the isolator mounts on the current rear. What are your thoughts? My last car I ran an 8.25 with no problems, but it made less power. I'm just not sure if I should get the 8.75 or spend the money elsewhere.