Considering anchor bolts will hold a sign 100'+ in the air with considerable wind load, I'd say they would fit the bill for this job.

Plan B............
 

Went with this design. It took 2 1/2 yds of high strength concrete plus some #4 rebar. When the existing slab was cut open, it proved to be a full 4" thick except for a void where I needed to mount a lift post. The hole was dug so that the new pour would lock under the existing slab on all sides. So now, I have a monolithic base block that weighs approximately 6500 lbs and is held in place by the rest of the slab. The new base is approximately 11 1/2 feet wide by 5 1/2 feet front to rear. Once I get the anchors properly set, I don't think there will be any problems with the posts staying put! The original contractor picked up the labor cost and I paid for the concrete. Might be the best money I ever spent!

Knowing something is dependable during procedure = Priceless.
Congrats to your efforts.

1. Steel subplate that is about 24" square under each foot.
2. Bolt or weld the existing foot to the subplate
3. Attach the subplate using multiple chemical anchors
4. Test before you get under a car

or a 48" deep sonotube.

Post concrete work cleanup...........
 

Cutting a major hole in a concrete slab makes a TON of concrete dust! Even though everything was covered (as well as possible) with plastic sheeting, everything got a good dusting of fine concrete powder. I have spent the last couple of days cleaning up using a shopvac, an airhose and a bucket of water w/wet rag to eliminate the mess. I've got two storage shelves to go and all will be back to "normal". The mess would have been worse except for the two ceiling fans and a box fan that blew most of the dust out the front door. At least now I can get in the shop without having to wear a face mask! A few more days and the new concrete will be at 70-80% strength and I can set the anchors for the lift posts (YEA)!