The first thing is the BIOS. I've been testing 5802 and I can't get it to boot into El Capitan with Clover at all. I used 5601 even before modifying it and was able to install Mavericks with Chimera and UniBeast just fine.
So, first get the BIOS 5601.
BIOS
Disable VT-d
Enable I/OAT in CPU IIO Bridge Configuration
Then download Mavericks.
Then use UniBeast with Chimera to install the OS using npci=0x2000 PCIRootUID=1.
Remember that Maxwell GPU's are not supported on Mavericks. So, definitely no 900 series cards and some 700 series cards.
I put the GPU in slot 3 and never had issues.
If you have a problem getting into the installer, you may need cpus=1.
After you have successfully installed the OS, use MultiBeast for Mavericks to install the rest of the kexts. Use EasyBeast, and select ALC 898, 3rd Party SATA, Intel Generic AHCI SATA, ElliottForceLegacyRTC, EvOreBoot, all FakeSMC plugins, NullCPUPowerManagement, USB 3.0 Universal, VoodooTSCSync 8 Core, AppleIntelE1000e v3.1.0, Chimera, PCI Root ID Fix, and either MacPro 5,1 or 6,1.
After all seems well, you can mod the BIOS chip by following the instructions here [video=youtube;fHAlsQvDMN4]https://www.youtube.com/watch?v=fHAlsQvDMN4[/video]
It's not hard but I would recommend having a second BIOS chip ready or you may be able to use the CrashFree BIOS method as outlined in the manual.
Let us know how it goes.
For those wanting to do this install on Clover, you should be ok. I'm still testing some things out to figure out why I have the GPU bugs I do. I'm thinking now, that its more related to my original install. I'll have a busy schedule this week but I will try to do a fresh install and see what happens.