The easiest way is to get a stock, 1971 (matches your engine) 340 converter. Depending on who you believe, the stall speed was between 2200-2400. I know the 71 340 Duster I bought new had a stall speed, if my memory serves me right, of around those numbers. For the flex plate, what's wrong with a stock Mopar one? Millions have been used by Chrysler. There are only 4 of them, 2 -11" ones and 2 -12" ones. The size is dependant on the converter, either its an 11" Converter (the ring gear is about 1" in width) and a 12" Converter, with a 1/2" wide ring gear. See the attached scan. Please note the mounting holes of the plate to the crankshaft.
I hope this helps you. With a mostly stock engine, I can see no reason to go to any other converter than a stock 340.
You should be able to get the flex plate from either your local Chrysler dealer or from any parts store like AutoZone etc. AutoZone I believe sells Daaco Converters, so they should be able to supply that to you also. If you've got any further questions, just ask.
