It is VERY EASY to damage your frame rails, easier than you might think. Find a SAFE way to USE WOOD BLOCKS to prevent damage. FWIW, I would NEVER (again) jack a Classic Mustang the ways I did for years and years, not since I had to repair this damage and much more like this on the front frame rails and areas forwards of the rear leaf springs, all of the places that I USED to use regularly before owning a Car Lift. Apparently, every time I jacked the car, I must have done more damage...and for the most part, I never realized just how much!
Use the floor jack to get the car up in the air. I use the engine crossmember for this purpose. ...
Personally, I would not do this (anymore, like I used to) either...but it is a way to raise the car!