7 1/4" info

I just busted the spider gears in the carrier of my 7 1/4" axle. It's behind a 3000 lb. '74 Dart with a mild 318(.440 cam, 600 carb and duals)and a built tranny(2500 stall, B.W. clutches, kevlar bands, a shift kit, koleen steels). I'm thinking of getting a 3.90 gearset and a spool if i can find it. If not, i'll probably just replace the spider gears and pop in the 3.90s. First of all, will switching from 2.76 gears to 3.90s help alot? Will I feel the difference? 2nd, does anyone make a spool for the 7 1/4" differential? 3rd, will my 7 1/4" hold up behind my configuration, or break soon after the rebuild? And last, how many splines does my 7 1/4" axle have? Is it 25 or 27? Or something completely different?
Lots of questions, I know haha. I would very much appreciate everyone's help with this!