TC flex plate

I have a 70 Duster currently has a 318 with a stock TC I had the 727 completely gone thru
two years ago when I put the 318 in it. I am about to put a 340 in mostly stock a 71 motor
but want to change out the TC the one with the 318 is pretty sluggish. I will be driving it during the summer as we have loads of snow here in the winter so mostly a daily driver might run it out once in a while. Any suggestions as to brand and stall speed, flex plate etc.