If you want to get it right, you should do it with the full weight of engine and transmission installed and the body in place. True, any aftermarket part company will sell a generic panhard bracket that is close to being correct, but they can not foresee all of the variables.
If you put about 300 pounds on each framerail at a point about level with where the firewall should be, that will be a close enough simulation of the total weight.
Call 2 fat friends over and have them stand on it, and no, its too far away for me to come over.