Most of the time, I do them together; easier to look everything over and deal with the flex plate/converter/clutch. I use a leveler but have done it the hard way when I had someone around to help. I cover the aprons, rad support & firewall with moving blankets, wrap a towel around the tailshaft, put a floorjack under the car where the trans is headed, and get the rear axle up on jack stands to improve the angle. I lay the headers against the blankets covering the aprons and bungee them up.