I put my 67 on a rolling carrier in order to paint the body color and black out.
Although I used 1 1/2 white PVC pipe you can use iron pipe. Here's a pic:
The lateral 2x6s are laid flat and are going from side to side. I used the holes in the frame there with great success and good balance.
I would suggest the following:
Use the rear hole in the front sub-frame that has a hole into the interior.
Take out the two post lift pads, put a large washer on the underside of lift arm, a pipe on top of the lift arm, a washer on the underside of the subfame and a washer on the top. Run a threaded rod through the whole mess and bolt it up.
To keep all of this from shifting weld smaller washers to the large washer so they index on and go into the lift arms and sub frame holes.
In the rear (as seen in my pic lined up with the other 2x6) there will also be frame holes. Instead of going all the way through they dead end. Use toggle bolts here.
The car was very stable on just these 4 points.
I'm sure you can get other ideas from this one and I'm sure others will chime in.