Thanks Per!
I got the port face finished tonight, the slots were milled out first and then the 10BA clearance holes were drilled and 5 were countersunk for mounting the plate to the block. It does call for the lower ones to be countersunk but as you will be able to see these I will use studs and nuts rather than countersunk screws. To make the countersink I first tried using a centre drill but it didn't work very well due to it being a little too small so I used a drill and carefully pecked down until the screw head was just flush. The ports then had the corners filed square as per the drawing.I then lapped both faces with 600 grit wet and dry to take off the machining marks. The DRO is again proving to be an excellent investment, all the bolt holes lined up perfectly, quite pleased with how this has come out