Replacing engine without removing the tranny?

Just wanted to see what the pro's and con's are with swapping my 273 out for my 340 without taking the transmission out with it. Tranny is rebuilt and works fine. I have a 727 that I will be freshining up, but for now will just keep the rebuilt 904 in there.
I have never done just the engine, always the engine and tranny together.
Does anybody have any tips they can share with me? Or is it more trouble to unhook the tranny?

Also, I purchased a engine lift plate that bolts to the intake manifold. It says it can be used for aluminum intakes as well. Is this safe to do or should I bolt the steel manifold on for installation purpose.

Thanks for any help.