Swap k-frame without dropping engine?

I would leave the engine in the car. I held my 273 above the K-frame while I painted (motor mounts off). I used 1/2"D rods in the big holes in the bottom of the block, but I recall they are only on the driver's side.

As mentioned, you can support the engine from a beam across the top. I have seen people use a 4x4 cut to fit across the top of the inner fenders. Maybe add some cross pieces to spread the force and not crimp the metal. If you have an engine lift, that is simplest. You can also support the engine from the oil pan on a wide board to spread the weight, but jam wood on the sides to the inner fender so the engine doesn't tilt over. The engine isn't as heavy as some imagine and I have heard of 4 big guys pulling one out by hand.