No personal experience, but I've heard nothing but bad things about airbagit.com and their products. Even the name "Denominator" seems like a rip-off of better known brands. I've heard nothing but good things about Slam Specialties on the other hand.
Have used both numerous times and never had an issue. If I had to pic o.e over the other of course it would be slam bags because of the internal bumpstops and the anti-ballooning feature, but have also used the denominator II with excelent results. I am current running the denominator II on the front of my 54 with slams out back.
Denominator brand comes from Joe Morrow (airbagit.com, AIM Industires, etc.). Please, before you spend one penny with him, do your research... his reputation should point you in the right direction.
I would highly recommend slam bags, but will never buy anything from airbagit.com again!! Parts were on the cheap side!
I run slam specialties on my car, and a few of my buddies also run slams. Only one issue with the threaded insert for the air line fitting, but only took a week to get the new one at no charge.
On your 53 it's all in what you want what 4 link what bags step notch if you what good parts I would go with gambino kustoms or Jimenez bro. They both have cool styles I'm planning to go all gambino
I would check with Gambino Kustoms there an alliance vendor on here. They can get you anything you need. I bought the notch and Fatman spindles for my car from them.
Slams are some of the best bags out there. They have internal bumpsteps and are tuogh as nails. Gambino can help you with everythig you need. and he's a HAMB'r to. You might get a discount.