Yep, counter sunk capscrew holes! I can't wait to finish the brackets off and bolt everything together.
Got to machine up the spacers and clean up some edges, but it fits perfectly and is really solid.
Here's my 3d model:
My friend at work had a piece of 7075 alloy lying around so we used that for the brackets.
I was going to get it all anodised black when finished but it looks so nice I might leave it bare alloy.
I'm going to make alternator brackets to match, with a little rod end hex turnbuckle underneath.
I did have to modify the body of the car to fit everything. The SPAL fans on the Griffin radiator clashed with the serpentine pulley of the PS pump. I cut the radiator mounting flanges off the radiator core support (I wasn't using them to mount the radiator anyway) so I could move the whole radiator assembly forward away from the motor by about 10mm. Have about 5mm clearance now, it's tight but it all fits.