Summary

Spacers to allow you to clear the dowel pins when bolting the engine to an engine stand. NA and NB miata, 1.6 and 1.8.
100% infill. PETG. Lots of top layers, bottom layers, and wall thickness. Use some washers to help spread the load. Be careful and don't drop your engine. This is plastic after all.

Entire spacer thickness is 8mm.
Spacers for the dowel pins are 20mm holes that are 6.5mm deep (for a 19mm pin that's 6.25mm long... expect a little bit of squish and deformation when everything is torqued down).
Holes for the bolts are 12mm.

Use a stack of washers for the non-dowel bolts to keep everything even.

Yes i'm using the head bolts to mount the engine to the stand. They are the right length and thread for this.