Thx peoples, in the end I used the two uppermost and two lowermost bell-housing bolt holes, I.e.:

My main concerns are about the stresses on the engine block bolt holes. The two lower holes will be in compression while the two upper holes (each 15 mm of 3/8" NC aluminum thread) will be in tension equal to more than twice the weight of the engine and many times more than what they could see in service or have been designed for?
The lower holes, one is in a skinny flange only designed to support the starter-motor?

When I spin the engine the bottom holes will be the top holes and compressive load will swap to tension, of even higher magnitude (due to the relative dimensions). To be safe I will not be spinning with the heads on.
I've interpreted from your advices that, for various reasons, alum V12's have had "supplementary support" of one form or another. So, I'm keeping the hoist in play, via an old fan-belt off the crank pulley, to relieve about half of the deflection that occurs when the stand is loaded:

Thanks again, appreciated.