I am building a 73 Roadrunner/Satellite it has the 8-1/4 rear end in it That I think was trashed pretty hard so I am going to replace it. What's the better rear end 8-3/4 or 9-1/4 I found both but I don't know which one to buy can anybody help me make a decision.

It really boils down to which one you can find with the correct axle width.

There are advantages to both. The 8.75 has a strong parts support and offers the ease of gear swaps without specialized tools in about an hour. The 9.25 is more common to find , much cheaper and in many opinions, as strong as the 8.75. The downside is that unless you have the skills or a friend with a axle shop, once you choose a gear ratio, it gets pricey to change gears. Both are adequate for a street/strip car up to 400 HP.