8 3/4 vs 8 1/4 weight

I have carried those rears around in the past when I was younger, and if the 8.25 was lighter, I didn't notice it lol. I mean, I'm sure the 8.25 is lighter, but not enough to make a difference to your engine, or to your mpgs. I mean think about it; 20/3500loaded= .0057, or .57%, yes decimal 57percent.

My unused fold-down rear seat was 64 pounds...... so I shelfed it.
One 15 x10 alloy slotmag is 14 pounds. Two alloy slotmags weigh about the same as one steel Rally, size for size.
Gas is about 6pounds per gallon, so a full tank is about 90 pounds.
From the headers back, I think my dual 3" exhaust is 72 pounds; half of which is the Dynomax muffs.

For a streeter, making the rear 20 pounds lighter is, IMO, a waste of time. plus it makes an already nose-heavy car , worse. I like to run the 8.75 under everything; simply for the gear swap-ability.
Just put a 2800convertor in her and your stocker will never care about whatever that weight difference might be.
I'll tell you one thing tho; 295/50-15s on 15 x 10 alloys at 24psi rides real nice like; compared to say; how I remember E70-14s Polyglass GoodYears,@30psi, on steel 5.5 Rallys. 275/50-15s on 8.5s@28 were maybe a lil better.