Welcome! As for your question on the stall speed for your torque converter, I'll just offer some general info. Some or all of it you may already know.
The 1970 Challenger factory stock converters were:
~2350 to 2650 RPM for a 383 4bbl engine
~2000 to 2300 RPM for a 440 4bbl engine
~2650 to 2850 RPM for a 426 Hemi engine
As for personal experience, I have a '70 R/T SE w/383 that I stroked to 426. I used a B&M Holeshot 2400 converter, part number 10415, advertised stall speed for this converter is 2300 to 2500 RPM. So it is basically a direct replacement for the factory converter. I have been driving it about 1000 miles per year for the last 8 years with absolutely no problems and can say that I am quite satisfied with it. This engine has a LOT of torque and the trans responds to that very firmly. Very easy to roast the tires if I so desire. I also have 3.55 gears in this car.
That being said, I also have a B body that I restored recently. It is a 499 stroker with A&A built 727 with a custom converter for this setup. This converter is a 2800 to 3000 RPM unit built by Transmission Specialties, part number 22312 XLS-XHD. While this is a VERY stout engine/converter/trans package, I can definitely feel the difference in the stall speeds. While I haven't driven this car anywhere near as much as the Challenger yet, I don't think I would enjoy a street car with a stall speed much higher than this one.
One final bit of advice, if you are having your transmission rebuilt, talk to your rebuilder about converters. They can guide you depending on all of the parameters of your car. As far as that goes, you can also call the converter builders directly and they will help you select the proper converter for your car. Good luck!
