Ordered a gear vendor, help with deciding gears

in my barracuda i
No cutting required in my 68 Barracuda. I hung it, jacked it into interference marked the spots, and pulled it down. Then massaged the high points. Did that two or three times until the main case hit the tunnel.Then dropped her down onto the later style 73 up cross member, which I had to slot out the thru-bolt mount just a lil.
I did not try the 68 crossmember, as I had long-ago converted to the 73-up style, with a poly-mount..
After driving it, I noticed a vibration. I went back and readjusted my pinion angle and all-done I was.
It was far easier than massaging my TTIs
Then I had to measure and order a shorter driveshaft...... which the first one was too long.So I moved the axle back. But then after the 7260 rear joint failed, I put a 1350 on the driveshaft and a 7290 on the pumpkin, then married them with an adapter joint, and of course the driveshaft had to be sent out again,lol. That's HotRodding.......