Just a thought, goigle up the VDO electronic speedo, and how it is wired in, i know of a few who have gone S14 elect speedo box output, into the VDO dash gauge, maybe there is a dongle/interface, they supply as a "multifit" , or a pulse count reducer/increaser dongle.
The back of the VDO has calibration covering all types.
Re the HUD, maybe ask around some of the circuit guys that use them.
Be interested to see what you discover, please post results!