IRS differential or 12 bolt???
 

I am trying to figure out what would be better for my needs. I am building a 69 Camaro just for the street, and I need a rear end that can handle 1000 HP. Is there a IRS differential built for high HP or should I go a different route?

12 bolt, hands down. If you're building the car for the street, you'll never see the benefits of IRS, but get to put up with all the down sides.

What are the down sides of a IRS?

And for the street, is a 12 bolt better with leaf springs, 3 link, or 4 link?

Cost, complexity, and packaging are the first that come to mind.

For the street you'll likely see very few differences between your suspension types. If you're looking to ditch the leaf springs there are quite a few bolt in 4 link packages available.