I have AFR alloy heads & they have a "spare threaded hole" at each end - I just made 2 x 6" long steel straps, bolted into these holes (front left / rear right) & ran a shackle thru each, connected to a chain running up to the engine crane (longer length of chain at front, shorter at the back). As my tranny was still connected, I ran some poly rope from the engine crane hook & around the rear/bottom of the tranny - a "spanish windlass" on this rope allowed for correct tensioning. When I jacked the whole lot up an inch or two, I then adjusted the "spanish windlass" so the everything just floated nicely, then used the crane to "up & out".
Zero technical sophistication, but it worked well.
If you don't have the spare threaded holes at the ends of the heads, I am sure that the same simple setup will work with the lifting straps bolted into the front/back header bolt holes.
Disclaimer : Don't lie under the engine until it is back down on solid blocks - flat 12's belong in Porsches, flat GT40 owners do not belong under their own cars !!!!
Kind Regards,
Peter D.