I just did this replacing my rear end and ALL suspension. I bought 2 lengths of 2x4 wood. I used 2 jack stands under the front subframe, then used 2 jack stands with the 2x4 under the rocker panel on each side as far to the rear as possible. The car was sturdy as hell so long as your front jacks are as far forward as you can get them so the back of the car doesn't lift when you remove the axle, springs, etc. I also have a couple of floor jacks I place here and there as insurance.
I'll be doing this again for my T56 swap in just a few weeks.
OK, retired construction engineer here. I made a similar setup with a couple suggested changes and an option. 1) Use 4x4's instead of 2 x 4's. Not a big cost difference and WAY less flex in the 4 x 4's. And these days, trying to find a 2 x 4 that is actually straight is a challenge.
2) Optional: I mounted my 4 x 4's on HD car skates and then placed screwed down 2 x 4 blocks at the top ends of the 4 x 4's under the front and rear of the rocker panels. You will need clearance for the dips in the floor panels - especially the rear floors. Thus I could actually move the body shell around and "park it" when I was done for the day.
3) One additional caution as anguilla mentioned: Support at the rearmost section of the rocker panels is a bit too close to the tipping point of my setup as shown here but with a full front clip and motor installed. Additional support under the engine or subframe is required.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI