The 4" frame tube is the only place I use. A low profile floor jack positioned in the center of 4" tube and floor jacks for support. Jacking the front is a little more difficult as you have to set the jack at an angle to clear the oil cooler inlet.
I also recommend the 4" frame tube. That said, be careful. The tube is round. Make sure you are well-centered on the tube before you jack, and make sure your jack pad isn't going to slip or skid on the tube. Last of all, and I'm sure you already know this, don't get under the car on a jack! Put some blocks or jackstands underneath -- definitely better safe than sorry!
I had mine slip off the jack once. Luckily the only casualty was a power steering pump fitting and hose -- creating a huge mess on my garage floor. Be safe!
There's a flat section of a 3 x 3 tube just inside the mounting bracket. I have used that as a jacking point on my car a number of times without any problems at all. Once the car is up in the air though I do always either put a wooden block or a jack stand under the 4" tube before I slide under.
I lift mine from there just move the jack up to the point where it attaches to the 4 round. Then put blocks under the round tube.
I removed the wheels from my floor jack to make it fit under easier, if you don't mind dragging it on the floor instead of wheeling it.
I jack from the front under a control arm, this gets me the clearance to put a jack stand under a frame rail. Then I jack from the cross-tube. In the rear, I jack under the pumpkin, with the jack at an angle, coming out under the car by the rear wheel.
I jack from the location indicated regularly using a low profile jack: it has never been a problem. BUT, I only jack the car high enough to get my caster plates under the wheels, which raises the car about 3" further off the ground. I can then get a conventional jack under the 4-inch tube rails to get the car high enough to receive my jackstands.
So it depends on why you want to get the car in the air. My racing jack on the spots that are highlighted works fine to change a tire. Otherwise, I use the above procedure. This chassis is plenty stiff enough to handle that off-center jacking point, IMO.
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