Novice needs help

Okay, More rookie mistakes... Crawled under the car to check to see the gear ratio tag & lo & behold, no oval 10 bolt with 2.94 gears but a hex 9 bolt w/ 2.76 gears! Didn't make me feel very warm & fuzzy.

Since I'm doing a bucks down build which should net just shy of 300hp (288hp by a member's desktop dyno), should I even worry about having a 7.25 back there? Just swap the ring & pinion to 3.21 & drive it until it does or doesn't explode. Or should i pull the trigger on the B-body 8.25 w/ SG and mod it to fit. If I go with the B-Body rear, what steps need to be taken to make it work? I've also tracked down a peg leg 8.25 out of an A-body a few hours away but have to check on bolt pattern & such.

Could use some advice here as I don't want go into panic mode, as I have a tendency to do, & make a bad call.