I went with the aluminum summit one. I would recommend the summit or griffin. I bought the summit 31" L X 19" H: part number SUM-380431. I couldn't be happier with how well it keeps temps low especially compared to the stock one. The only issue I had was the fitment, it fits in there tight (had to bend some metal lines on the passenger side but nothing serious) and because its almost 3 inches wide, the fan shroud (assuming you still have the stock one) will move closer to the fan belt. I ended up cutting the stock shroud when I had to get to the belt tensioner. So, I would recommend going one size smaller, summit makes a 27 3/8" L X 19" H part number SUM-380428. I think this may have made the install a lot easier. The other issue you will have is connecting the hoses. The inlet and outlet of the summit ones are bigger than the stock ones; I used some Vaseline to help get them on. I also cut about two inches off the bottom hose; u will notice that the hose will bend slightly due to the bottom inlet slanting up-word as well as the radiator being wider than the stock one. You will need to get a fitting to go into the radiator neck that connects to the overflow tank hose. I got this from pep boys. Also, you will have to drill holes for the shroud to hook up at the top, I used the stock shroud screws. Then drilled holes for the shroud at the bottom of the radiator; I used zip ties to hold it together at the bottom. The only other suggestion I have is to consider installing an electric fan while doing the new radiator; if you do it later, it will mean more work later on. The only thing is without the stock fan shroud, you will have to figure out a way to mount the stock over flow tank or buy an aftermarket one and mount that some how. Also, there is no drain plug on the radiator, I decided I wasnt going to spend another 100+ for a more expensive radiator with one. I don't think the griffin or jegs have them either.
Another thing to think about when deciding on the summit or griffin, is to look at the construction. The summit is fully welded while the griffin uses an epoxy seal. Most people I talked to told me to get a welded radiator. I did a little research on epoxy ones, but i'm still not too sure how they compare. During my install, I first thought I should have spent the extra money on an actual direct fit mustang radiator, but as soon as it was in I was glad I went with the summit one. Plus I was going to buy an electric fan with the money I saved but never got to it.
I know this was long, but you should have no problems with your install and it should save you some money, which is always good. Like most things, its better to do it right the first time. I learned a lot from my experience and I hope all this helps.