Good to ask! If
Always good to question people about this. Here's the detail on my reasoning. Let's figure it's a 1000 lb engine just for comfort. Let's also say it's 2' long with the center of it 1' from the stand. This gives us two basic forces on the stand, a downward force of 1000# and a torque of 1000 lb-ft. The downward force is pretty easy, just figure it's 1000#/4 bolts so 250# of shear per bolt. The torque is a little more difficult. Has a lot to do with the spread of the bolts on the tranny flange. Let's say you get them 9" apart from the upper ones to the lower ones. The lower ones are getting all compression, nothing for the bolts to do except the shear from the weight. The upper ones are trying to get ripped out though. 1000lb-ft/9" (.75 ft) = 1333#, but there are two of them so 667# of tension each plus 250# of shear each. sqrt(667²+250²)=712#. Then we have the 3/8-16 bolts. Tap drill for that is 5/16, so: .312²*3.1416/4 = .0765 in² of steel. Now figure it's really crappy steel, worse than some aluminum, and has an ultimate tensile strength of 40000 ksi. 40000*.0765 = 3058# to break the screw against the 712# you'll put on it with a 1000# engine. So you've got a safety factor of 4+ which should be just fine to handle head bolt torquing etc. The important things here like in my original note are getting that spread, you can see how if 9" went to 4.5" you'd double the tension, and getting enough thread engagement so you don't pull the threads out. 1-1/2x diameter or greater is a good guideline. I hope this helps.