Dirk,
Oh man, this is a six beer (or more) discussion if I ever heard one....
Coincidentally I bought two torque converters from FTI yesterday. Here's his URL:
http://www.fticonverters.com/
The FTI converters are 9.5" 1600 stall speed units. I have a 521 in the Cobra and am putting a 557 in a Zephyr (Fairmont badge twin) station wagon, both with C6s. They each weigh around 3200 pounds and have hydraulic roller cams, good to ~6000rpm. The wagon has a 2.73 and the Cobra a 2.88.
The problem with the Cobra has been overheating the trans when running at Sebring with a 9.5" Edge converter which apparently has too high a stall speed and not enough
oil flow for proper cooling. It melted the fan on my B&M cooler during one run....
The FTI converters will have "mud truck" specs to prevent overheating during 30 minute lapping sessions on road race tracks.
On the street, no problems, ever, with any of the three converters I've used. Well, I did have a TCI 12" converter which I think failed but that was years ago.
Here's a sample of why this is a lengthy discussion:
http://www.tciauto.com/Products/Tech..._explained.asp
If you Google "torque converter" and "stall speed" you'll get many more like this - not something you learn in a minute, sorry! BTW, just for grins, stall speed depends on the engine, the converter and the load on the engine which varies with speed, traction and who knows what else?
And you can see this will involve the whole car, not just the trans/rear ratio.
BTW, according to FTI, some of the "conventional wisdom" about how to achieve correct stall speeds is wrong. I recall someone years ago telling me "It ain't what you know that gets you in trouble, it's what you know that ain't so!"
The approach I'm taking now is to use the converter builder's experience and take his recommendation. I know, I did that with Edge and regret it, but maybe this guy does have the answer I need!
I don't have any idle problems with a lower stall converter and the whole car feels "crisper." Unless your car weighs more than mine I wouldn't recommend a 3.55 unless you like your engine turning pretty high RPM on the highway. With the 2.88 I can spin the street tires by whacking it at 100mph (try this on the track only, please) so having a higher ratio rear will just increase wheelspin a lot at almost any speed, and make cruising less fun in my view. With slicks I can use the throttle to steer the car on the track so it's really fun!
Hope this helps,
Tom