The new MB is a Gigabyte Z170XP-SLI. I install my tried and true EVGA 8800 GT video cards, but I don't have the option to SLI them. I have the latest 341.98 drivers. 
I may not be able to SLI because the 8800 BIOS does not support UEFI MB bios and there doesn't seem to be a way to disable it or tho use CSM on the MB. I only found setting to disable UEFI against a boot device. I don't see a bios update on this site for the 8800 to allow SLI with a UEFI MB BIOS.
I think my conclusions are correct and i'm really just looking for confirmation.
Might be time to upgrade my video but these card work so damn well I hate to get rid of them.  

hhmmmm I dont think legacy VBIOS on your cards would prevent SLI. Have you tested a second SLI bridge? Have you made sure that both lanes you have the cards installed on are at least x8?

If both of your cards are running at x8 pci-e 3.0 each then you could try this to get your sli working.

little curious why your using 8800GT SLI still for a modern computer anyway. An 8800GT SLI is slower than
 
GT 730 GDDR5, GT 640 GDDR5, GTX 550Ti
 
All of these cards are under 100 dollars