I built a robust cradle from scrap 4"x4"x10' forklift skid beams and some of the 3/4" thick planks. One can construct a sturdy cradle from wood, use a tire, which I have for the 383 short block I snagged this summer, or a purpose built one from sundry suppliers. I usually think of a cradle as something one will want to RUN an engine on before dropping it into a car.
I plan to buy a good engine stand to start work on the two motors I've got, as my cradle is meant for RUNNING the motor, not necessarily building on it. I will bolt up the radiator, some instruments, a gas tank and battery to the cradle too when the time comes to do so....