I used the Guide and successfully installed HS 10.13.6 to a 9010MT with i3-3220, Radeon 5450 (512MB) in 'legacy boot' mode (legacy ROM enabled and Serial Port disabled in BIOS). I didn't use UEFI for other reasons.
One issue I had is MultiBeast 10.4 would not install bootloader onto the hard drive (legacy boot mode). I'd seen the same issue using Clover while experimenting with a Vostro 400 (aka Inspiron 530). I used Rehabman's r4701 Clover to successfully install the bootloader. Afterwards, used the Unibeast USB to boot up the HS on the hard disk, and finally used MultiBeast to install Clover.
USB 3 sockets seem to be working. Applied FakeID to get the Radeon 5450 working. Used the 'free' app ResXtreme to fix the colour flickering issue instead of the 'limited trial' SwitchResX app.
One issue is I have not been able to get HDMI audio to work. I am using Voodoo instead of Lilu. I can choose HDMI audio as the output device when using Voodoo, but there is no sound. Lilu doesn't offer HDMI sound option. I'm having to use VGA and audio line-out as a temporary workaround. I've also noticed the voodoo volume level is very low and fluctuates too, so I switched to Lilu which is much better.
[Partially solved, Dec 2018]: One of the HDMI sockets on the monitor is marked for DVI use. I plugged the HDMI cable into this socket. I now have analogue audio from the Dell's Line-Out socket successfully feeding into the monitor. Screen was blank (no signal) when waking from sleep if using HDMI, but worked fine with VGA.... Using ATI injection with Fake ID:0x68E01002. Replacing FB Name: 'Eulemur' by typing in 'Cedar' resolved the HDMI wake problem.
For anyone who wants to put a 7010/9010 MT/DT motherboard into a 3rd party case, fixes for some of the 'Alert!' POST messages are described here:
7010/9010 MT/DT 'Alert! Front I/O Cable failure', and 'Alert! Air temperature sensor not detected' fixes:
https://linustechtips.com/main/topic/987783-optiplex-70109010-front-io-pinout/
7010/9010 'Alert! Power Button Cable failure' fix:
https://linustechtips.com/main/topic/925354-optiplex-7010-power-pinout/