You should do them all at the same time. And you need to remove the front bumper so the subframe will drop down enough (at least you do on a 67/68). Do a search here and you will find alot of discussion on replacing body bushings. I did mine last winter. It's a pain and takes some patience, and a couple of floor jacks and a bunch of jack stands. Here's a synopsis: Put some big mother jack stands under the rear axle. Then jack up the front and put some jack stands under the flat areas of the body near the fronts of the doors. Then loosen all the bolts (use lots of liquid wrench, take out the front seats to reach the tops of the rear bolts). Do the rear mounts first by lifting the front and pivoting the rear of the subframe down. Assemble them loosly. Then drop the car onto the jackstands under the front of the body so the front of the subframe drops down and do the front 4 mounts. You may need to pry the frame down to get the new mounts in. Have fun and take your time! Oh yeah, you need to disassemble the rag joint or loosen the steering column and the steering box. 35 ft-lbs on the front and 85 ft-lbs on the middle and rear bolts (you won't find that in any of the manuals).