Flex Plate?

I swapping Motors from an la 318 to a Magnum 360 in my race car and was wondering what flex plate do I need? The trans is a 904 I have the factory flex plate from the 360 it was in a 4x4 truck if that matters, the converter is a dynamic 8 inch with the lager bolts. I heard that i could use the factory one if i drill out the holes and oblong one of them. I'm planning on running mid to low 11's so i didn't know if the factory one would hold up.