Here's the platform that I made.
I think it could be adapted to a trolley jack using Mitch Leland's method even though I used an ATV jack.
If I were to give extra advise, I'd say strap the engine to the platform with a tie-down once it is lowered a little bit.
Also, use 4 jackstands to get the car level when you're dropping and re-installing the motor. This will make it easy to remove and re-install the bolts with remarkable ease.
Finally, you might want to use a second jack at the transmission nose to make sure that the shift rod clears the center tunnel without putting pressure on the shift rod. Once is is clear, begin lowering.
Credit for this design goes to Mr. Fred Cook, and the 2"x4" block supports the transmission at the point where the part number is stamped in the blocky aluminum protrusion at the bottom of the casting. This will allow you to support the engine and transmission as a single assembly, right under the balance point that Gunter mentions.