8 3/4 and you can swap gears Friday after work for races Saturday, and put the street gears back for your commute Sunday afternoon. What better excuse to get out to the garage with some friends with pizza and beers?
But back to your question. If you must change rear ends, now would be the time to go to an 8.75, and most people here will recommend that. You don't want to put much money into the 7-1/4. However, several times on this site, I have seen 7-1/4 rear ends (drum-to-drum) offered for free. Of course shipping is something else.
