Converter stall rpm?

Idealy you wast to be about 500rpm below your peak torque in your power band.
I make about 475lb/ft of torque at 4300rpm and the converter in my C4 stalls at about 3900rpm on the trans brake. Where you stall will also depend on the power you are making. For example, a converter behind a 500hp engine stalls at 4000rpm. The same converter behind a 600hp engine will stall higher.
You can also "loosen" or "tighten" the converter to the point were you will be going through the traps at your max rpm or shift point in third gear.
I just bought a 9 inch, steel stator converter with a 3/8 inch thick billit cover. A very nice but expensive piece at $1350. I had a 10 inch aluminum stator converter but I busted the stator on the launch a few weeks back. I was sick and tired of pulling the tranny for the few problems I had with it so I went a bit overkill with the converter so I wouldn't have to worry about it ever again.
Give Ian at Pro-West Converters a call. They are in Port- Coquitlam BC. He can get you set up with what you need.
His toll free number is 1-800-667-8884.
I can also e-mail you one of their applications if you want. So you can see what info they need in order to get the proper stall.

"Because I need 4000rpm or more rpm for my car not to bog"

Crist, how big is the cam in that engine? It sounds like you may have other issues.....

My guess would be because of the long runner intake on a fuel injected Mustang. It will in most cases make considerably more low end torque. Stick a box style intake on a fuel injected car and it would make more top end power but sacrifice low end torque. Steeper gears can help to a point were traction becomes an issue. But Jim said he already has "steep" gears. Are the 4.10 or 4.30's?
I run 3.73 gears. I didn't really like the fact I was turning 8000rpm at the end of the quarter with 4.10 gears.

If you have 4.10's and are just planning on running it on the motor alone you will probably be fine.
I only ran into problems when I fed it with nitrous. The converter slips with more power like I stated earlier. So it also revs higher at the end of the track. So it was ok at 7000 with 550hp but with an aditional 225 it would go to 8000 at the end of the quarter.