I have included a pic of using the scriber set up and the way I rotated the chuck with an 8'' shifter tight on one jaw until it touched the bed and scribed a line and turned 90deg and repeated. I did the full rotation to check accuracy and it was the thickness of the scribe line off but I dealt with that easily.
Pic 54 The scriber at work down the sides.
Pic 55 Drilling the holes for the head studs, water outlet and valve guides in the drill press after truing it up (again)
Pic 56 The head so far with the valve guide holes, the ignitor hole, exhaust port drilled and tapped to 1/4NPT. The inlet has not been finished because it has a pilot drill broken in it at present (the reason for walking away and writing this up) and the 2 holes in the raised centre will have the valve seats machined into them.
That might have to do for today, time for a beer.