Engine Swap Help Needed: 1964 273 to 2001 5.9 Magnum 360

Now I am stuck. Bought a weighted torque converter thinking this would bypass the flexplate issue and still get the external balancing done. Maybe this is old news to some, but the balanced TC has 22 splines and the old 904 trans had 18 splines. It goes right into place but the car wouldn't go into gear. It couldn't activate the pump.

I need to know if I can modify a 1999 flexplate by elongating the holes to make it fit with my original torque converter. I can't seem to find anyone who makes one for a 2001 engine.

Please help if you have any ideas. I am too far into to turn around now. One option thrown out by a local tranny guy was to modify the 904 to accept and change it to a 22 spline. Didn't sound like a very cheap option IMO.