I've read a lot of great information in the forums related to bushing replacement while the car is assembled. There are several references to a link to detailed instructions that I can't seem to get to. I have a 69 with the front clip removed. The entire drive train is still installed and the body is stripped to the shell. I have checked the frame for alignment front to rear (dim F) and corner to corner (dim E) and it is with in 1/16 in all directions. If I loosen one side at a time to maintain alignment, where do I support the body? Also, is it necessary to support the subframe any where on the loose side to keep it from dropping or twisting, say at the cross member? Thanks for your help!

Support the body under the front of the rockers, they will hold it fine.
Most likely, when you loosen and lower one side, the sub will shift, a lot, mine did. Just remeasure and adjust when done.