I have to replace the flex plate of my 81 300SD.

History:
- car purchased not running
-- engine runs
-- six bolts between flex plate and torque converter sheared off
- previous owner claims transmission recently rebuilt, but has no details
-- transmission looks clean for having 292K miles on odometer
- removed tranny
-- shrapnel in bell housing
-- a few sheared off bolt heads
-- torque converter spins freely on input shaft
-- output shaft spins freely out of park, locks in park
-- fluid is red and free of debris

Questions:
- any tricks to removing and replacing the flex plate?
-- is there a specified tightening sequence or can I tighten every 5th bolt until I've tightened all 12? I have the torque and rotation tightening specs.
-- is there a key or clocking device to be aware of?
- what caused this?
-- bolts overtightened
-- torque converter locked up (looks fine, but what do I know?)
-- none of the bolts backed out intact because there are 6 tips in the torque converter
- anyone got a flex plate for sale?

Thanks,
Sixto
91 300SE
81 300SD