They normally go in OK.

If you have access to a bender you can tweak it here and there to fit cleaner though.

Done a few, would rather build from scratch, but it saves money to buy the kit.

S&W Race cars builds a decent kit too.

(edit)- If you decide to go through the trouble of putting a cage in a car, go ahead and purchase the latest NHRA rulebook and follow it to a "T". If you can pass NHRA inspection and get it certified, you are golden anywhere. It also makes the car a much easier sell if you ever want to get rid of it.

I see countless people order a cage kit, stick, (arc), weld it in without following any guidlines on how and where everything goes in. It's a waste of time and money, and it's unsafe.