The solution I came up with was to get hold of a piece of steel plate the same size and shape (250mm x 150mm x 12mm) as that used on my engine stand. This was then drilled with 4 x M8 holes in a 70mm x 38mm rectangle in the centre of the plate to correspond with the 4 mounting points on the block. I then offered up my plate to the engine stand and marked out the mounting slots onto my plate using a blue Sharpie. I then drilled out 4 x M12 holes in a larger rectangle approximately halfway between the M8 rectangle and the edge of the plate. I stress that all this drilling work was carried out on my trust Startrite Mercury pillar drill. I wouldn't have attempted it without a decent pillar drill, as I doubt the accuracy would be good enough trying it by hand and drilling large-ish holes in 12mm steel plate requires that it's clamped down solidly to avoid the possibility of personal injury.
Before doing this I'd approached a local engineering firm with a 300mm long piece of 25mm round steel bar and asked them to centre drill it to 8mm from one end and 12mm from the other end, meeting in the middle, so I effectively had 150mm of bar with a 12mm hole and 150mm of bar with an 8mm hole. I then cut 4 slices off the M8 end of the bar, each of 12mm depth and 4 slices off the M12 end of the bar, each of 10mm depth. Again, I wouldn't have attempted this freehand using a hacksaw or angle grinder with slitting disc, as I couldn't guaranteed repeatability. But as I have a horizontal bandsaw I could get all the slices as close to the same depth as made no difference. With this done I bolted the slices to the plate in the correct orientation and took it down to some excellent and friendly welders I know. My welding is nowhere near good enough for stuff like this.
Results are shown below. Next thing is to get it plated to minimise rusting and it will be ready to use.
