After a lot of online research and trying many possible solutions, I've come to the conclusion that it's not possible to use a legacy PCI graphics card as the primary video card on a Dell T7910 workstation if you must have Secure Boot enabled. The reason is not a defect in the Dell BIOS, but rather the fact that with Secure Boot enabled, you need the graphics card to have UEFI support and, to my knowledge, no legacy PCI graphics card has been updated to add UEFI support. I tinkered with GPU-z, GOPupd, and amdvbflash and "added" UEFI capability to my PCI card as confirmed by GPU-Z. After this, I was able to boot Windows or Ubuntu using this as the primary graphics card. However, within minutes, the machine would completely freeze and had to be restarted. The Dell BIOS still saw this as a PCI Bridge and not a video card, but the OS was able to boot until it crashed/froze. I tried this multiple times with the same result.  You can use any PCI graphics card as the primary video card PROVIDED you enable Legacy Option ROMs and turn off Secure Boot. If you can live with this less secure set up, you can use a legacy PCI graphics card to drive your monitor. Just make sure to set the Primary Video Slot as slot 5 under the BIOS Video settings. Slot 5 is the only legacy PCI slot on this workstation.
I’m trying to use a PCI (not PCI Express) video card in slot 5 to drive a display/monitor. The reason I want to go with an older PCI video card in slot 5 (the only PCI slot) is that I want to free up all the PCIe slots for additional cards such as GPUs for machine learning.  I installed a VisionTek 900608 AMD Radeon HD 7350 PCI 512MB DMS59 Graphics Video Card in slot 5 and connected a monitor through a DisplayPort cable. The cable works as I’ve used it successfully with another video card. However, I see no signal to the monitor. I waited five minutes for the Dell logo to appear but nothing but a blank screen. Next, I inserted a PCIe video card in slot 2 and connected to the monitor using the same DisplayPort cable. The Dell logo came up and I entered the bios. Under video settings on the A34 bios (the most recent version), I set the primary boot video device as slot 5. Next, I shut down the machine and reinserted the PCI video card into slot 5.  But I still can’t see any video signal on the monitor connected to the slot 5 video card.  Any suggestions for getting the PCI video card to work in slot 5?
Dell Precision T7910 support for PCI video cards