My thoughts, and experience for what it is. Mig Welding on a table with nothing in the way, and using clean new metal, is a skill that does not take too much time to master. Welding a cage in a car upside down, getting sparks down your neck, and trying to get the undercoat and paint off well enough for a good weld is a lot bigger challenge. First is look at the cage two ways, making the car safe, NHRA rules lay that out, second, making the chassis stronger and stiffer without adding too much weight is the next challenge. I always weld on jack stands, car leveled, and make sure the doors still work and gaps fit well during the process. For most cars and home building mild steel is the way I would go. Moly is lighter and stronger, but also more brittle, and higher level welding skill is required. I have built several cages over the years, I remember one that when welding in the frame connectors, I got a spark down my shirt, and in pushing the creeper to try and get away from it, I hit one of the jack stands, and to my surprise, it simply slid out form under the car, and the car just sat there on three jackstands. I took that to be a test that the car was square and stiff as it needed to be.

I have a 64 dodge NSS car that we built at the tech school where I retired from as a teacher. The last project the students helped me with on the car was a moly cage. The welding instructor who was helping made the students fit the joints so a .010 feeler gauge would not fit in the joints before he would let them be welded. The welding instructor did most of the upside down, curled into a ball welding. My issues is as a 64 years old, I can not see well enough to weld in those positions anymore. Moly cage saved 100lbs, and made the car a lot more consistent, in 60 foot times. Since moly is so much lighter I added a little tubing here and there to make it stiff. The other issue was the original cage was built as a leaf spring car and over the years had been changed to a 4 link. The cage had bars were they were not needed, and needed bars in other places. The redo, fixed all those issues. And I was proud that the NHRA tech inspector who certified the new cage commented on the good quality of the job.

The other issue is resale value, if it is a good job, a moly cage car will always bring better money on resale.