ATX PS in OptiPlex 780...

hi all!!! FNG here.

I have an older OptiPlex 780 (service code 8X32KN1) dell support told me a standard ATX PS would fit. So I ordered a Diablotek DA 500 and found that it is too big to fit in the Dell case. More importantly the 20/24 pin main connector for the MB is too big too.  Is there an adaptor to make the PS plug into the MB?

Thanks for your help in advance