I just installed 6x9's in the rear deck today. I went with Infinity 9603i's.
1. That Howto was extremely helpful. Many thanks to dbaudiopro.
2. I had to enlarge the holes to get the Infinity's to fit. It was really too snug for them to drop in. I even took the XTC foam baffles off thinking they were the problem. Nope, the baskets on the Infinity's just wouldn't fit.
I ended up taking a small mallet with a rubber end and actually modding (read; beating sheetmetal) the holes.
It just took some extra effort. It wasn't really hard after I concluded it was the only way to get the Infinity's to fit.
You will find a lot of helpful info in the Audio/ Video forum like dabirdz96 mentioned.