'66 Convertible-would Like To Stiffen Frame-help!

This happens on the convertibles over time as the metal gets thin from corrosion and fatigue. You can get sub frame connectors from several vendors that will fit or if you or a buddy can weld you can do what I did for my cousins vert and buy 1 inch square tubing and some flat stock and make your own. I'm getting ready to do a buddies car in a few weeks.
Now that wont necessarily cure your problem because that sag can come from several weak points I.E. rear frame rails where they begin to go up into the wheel well, inner/outer rockers, floor pans etc.
Here is an example connector from CJ Pony Parts http://www.cjponyparts.com/sub-frame-connector-bolt-on-pair-convertible-1965-1970/p/SFC38/
Before welding these on you need to take the pressure off the doors by slightly jacking the car up from the middle. If you have two jacks (one on each side) is better or one in the middle on the support plate that is bolted to the bottom of the car.
Jack until you can open and close the doors with out binding. At this point (no binding) check door alignment visually and adjust gaps and lines. Two jacks are better because you can manipulate each side individually. Once you are satisfied then tack weld your connectors in place just enough to be secure and then release your jacks slowly and check your sag. You may have to cut the weld (reason for only tack welding)and take a little more pressure off the car to get it right. Then finish welding.