FYI to anyone who finds this thread, I ended up having to buy a custom torque converter from TCI to make this work. The flex plate I goty from mancini was not suitable for my application. I have found several threads on line with people telling you how to make this swap and none of them seam to have this problem. I have 4 torque converters sitting in my garage. One from a 65 cable shift BB 727 and two from SB 727s (one 19 spline and one 23 spline). They are all identical as far as torque convertor bolt dimensions. None of them would have worked with the Mancini plate. When I bought my custom convertor from TCI it was dimensioned the same as the others as well. I had to send it back to TCI and have them move the lugs on the TQ to suit the flex plate.
On a related note, make sure you have the lugs of the TQ 7/16" holes. If you go with 5/16" it will not work with the flexplate unless you by the shim kit that Mancini sells for $30 + shipping. The added thickness of the shims means that you lose thread contact when you do bolt the flex plate to the TQ. This could easily cause stripped threads. I had it happen twice. I eventually took the TQ to a machine shop and had it drilled and tapped to 7/16". I could have saved $40 on the shims and $40 on machine work if I had only have the 7/16" lugs from the start.