Dust Cover Question?

I went to install my dust cover last night and the dust cover that came with the bell housing (were separated at the time) 10.5" bell housing. The bolt holes that are towards the engine side measure 15 3/4" across, which are not even close to bolt up. Do I have a dust cover for a big block or for an 11" bell housing?
Not sure what P/N the bell housing is. Mounted on 340.
Any ideas which dust cover I need? Or if anyone has one for sale?

Thank you