9,1 iMac GPU upgrade

I replaced the NVIDIA 130GT w/ a 4850 AMD Radeon card from a 27" imac, same year (late 2009). Computer boots, all 4 LEDs go on, computer accessible from remote screensharing on the LAN so is obviously up and running. No picture on the screen (backlighting working).

I think it's an EFI/firmware problem w/ the new card. Any ideas as to a work-around? Edit the BIOS in the GPU to match the 24 inch screen or the Logic board ID? Thoughts? Thanks!

It starts fine, but no image on the screen. Clearly think it's a GPU-CPU issue, but obviously they are talking or all 4 LED's wouldn't light up....so it must be something written into the EFI that's preventing screen imaging, I'm thinking.