I would call Jason Coan at Coan Engineering, He will build you what you need, and the convertor in the Nitro Flash Truck stalls 4500 to 4700, but he might tighten it up a little. Any lower than that and we are getting down in to peak torque. The key is to run the convertor around the beginning of peak hp curve, this allows you the most time going down the track with in the peak power curve.
And a good torque convertor builder doesn’t rate a convertor at a given stall, he makes a good educated decision, and you adjust it a few times from there, until you see where it sets in high gear on the graph.