I went through this a few years ago. No one makes a one piece gasket for the 340.The three piece works fine if properly installed. Superformance is definitely good stuff.

I am a little embarrassed to say that I tried to make my own one piece 340 oil pan gasket by gluing (using right stuff or silicone or something) the 340 front and rear pan pieces to a modern 5.9 rubber reinforced pan gasket that I cut apart but it didn't work. Not precise enough and too much glue. I got the idea from a magazine article. I tried to copy it but whoever it was had way more skill than me if they were able to successfully bond it all together and get it to work without leaking.