Torque converter help.
If you're running a 727 behind that 340, you don't need an aftermarket flexplate for what you're wanting to do. An aftermarket converter will bolt right up. I would recommend picking up some ARP torque converter bolts while you're at it though.
From experience, it's not THAT much more expensive to contact a converter company (PTC, Hughes, TCI,etc) to see what they would recommend for your combo. A converter built for YOUR car makes a world of difference.