Well, it's not that simple. You're comparing an EFI intake to a carb setup. AFAIK, the stock intake is good for a doorstop compared to what's in the aftermarket. A Cobra intake is supposedly good up to 400 horse (n/a of course), just for comparison, so I'd draw the line on the stocker at a little less than 300 or so. I'm sure there's exceptions (there are to EVERY rule), but usually the stock intake, heads, and exhaust are the first things to go on a 5.0L Mustang. They're adequate pieces in stock form, but that's not what the majority of us want.
As for a Stealth, I've heard good things so far. Here's the question that should determine if you want to convert to carb: will you drive this car every day, and/or in a lot of different traffic situations, and/or year 'round? If yes to any of those, stick with the EFI, if for nothing else than driveability. Carb guys don't attack me on this, but we all know that EFI is more suited to daily driving and changing atmospheric conditions than carbs are. In addition, until you upgrade your cam, heads, and exhaust, it won't yield any more power with a carb. In fact, you may lose power with that large of a carb on a stock 5.0L. My suggestion is to stick with the EFI.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch