How to use K8200 mainboard with K8400/Vertex hardware?

I would like to ask how to switch mainboards between K8200 and K8400.
Mainly how to set the K8200 up for use with the Vertex hardware.

Most of it is straight forward, which it should be as both are specified as "Motherboard 77", but some things do not line up and att least one thing must be changed around quite a lot.

The main problem is to move controller fan [FAN1] to the pins of the bed heater - which in it self is the first problem.

H3 6 HEATER_2_PIN //BED
H6 9 HEATER_BED_PIN // BED

Which is the correct one?

And why is H3 named 6 (etc), instead of according to ATmega2560 and the schematic for the K8200?

Which is a question that goes for all pin numberings, especially those set for analogue.
But not the actual issue here.

Another problem is to get endstops and directions to work properly.
It seem to originate in different microsteps in the two variants of NEMA17 used.

I would be ever so grateful to get this sorted.

Why I want to do this?

It is more a need, I need to use PVA and it is impossible to push that filament through the 700mm Bowden of the Vertex.
So my K8200 get the K8400 mainboard and a twin extruder, non-Bowden twin extruder.
But to make that happen I need one printer functional - while putting the "upgraded" one together.
And the Vertex is also a quite nifty little machine to have around...