Breaking Flex plates

If it's a earlier 318 crank,it might have the smaller pilot hole.
The pilot hole is where the torque converters center hub sits and is held, to stabilize the converter.
If you have too early of a 318 crank, then it is possible that you have a more modern converter snout with an older crank that would need to be opened up to the bigger size.
Measure the crank pilot hole in the back of the crank and then the converter snout(center hub)
Call dynamic transmission and talk to them about what the cause could be, and mention what i just said.
www.dynamicconverters.com phone#(302)454-9203