You should look into the amount of energy stored in your self termed "bomb" at 3k psi. I think you'll find it quite sufficient to keep you from cradling the tank between your legs as they fill it. But then again, I'm in the crowd that believes these tanks are dangerous, and everyone knows that's a crazy theory
I'm glad you found a shop that would fill your tank, and feel free to do what you want with it, but my advice would be to sell it now and wait for a good deal on a newer tank. But, as you've seen, lots of people with those SLC prone tanks, who have years and years of experience with their own set of tanks without any issue, and who feel that with yearly VIP's and visual eddy, any developing cracks can be caught before they are an issue, and the extra $100 which would buy them a new tank is too much to spend to make the risk of an SLC explosion near 0, and that those of us who feel cracks can go undetected, either due to the nature of the crack, or poor checking practices by dive shops that don't take proper care, or just miss the crack on accident, or develop too fast for a test to catch it, and who would rather buy newer tanks to virtually eliminate the risk, are crazy.
I won't lecture you one way or the other, adults can make their own informed decisions, both you and your dive shop.
Please be advised that cave divers are NOT cave filling steel 72's to the typical "cave fill" pressure of 3600ish psi. As such, there is not much anecdotal evidence showing it to be safe. As a fill station operator, I would not overfill your steel 72. In fact, you might encounter trouble even getting the 10% overfill to 2475 due to trouble tracking down the REE number.
Have you checked to be sure the tank has no liner inside?
I don't mean to be condescending, but here's my suggestion: Do your research before purchasing dive gear. If you don't know enough to tell the manufacturer or the valve type, you run a greater risk of buying a tank that won't work out well for you. I bought two 6351 tanks before learning my lesson, and then I research and researched and researched. Now, I know what to look for. It helps me know when a good deal is to be found, and to bargain the seller down.