Flexplate Bolt Sizes

I have a 74 Duster with a 318 that was originally a automatic car, that i am putting a 3 speed manual in. I purchased pressure plate bolts that are wayyyy to small, then thinking about it I dug out my old flexplate bolts that are the right size but too short.....

Does anyone know what size this is and where i can find them?

May just need to run down to the hardware store these bolts are only grade 8