cracks in the floor

Sadly - I have 2 cracks in my floorboard. . .Im not ALL that surprised, the car is an 89 and has never had sub frame connectors.
DILEMA- I know want to add sub frame connectors to stop any further cracking. Ive asked, and have had 2 totally different answers. The question is "Should I mount the sub frame seat cross bracket?"
School of thought 1 - If i mount the "under the seat bracket" I might cause further cracks (cracks are currently on the outer portion of both driver and passenger seats, think near the doors). That is what one shop told me- that it would do further damage and that it would be more expensive to add anyway.
School of thought 2- If i don't mount the aforementioned bracket , id actually save a couple hundred bucks (always good) , but id not be securing the already cracking floorboard. Sure the sub frame will be nicely tied up , but the seat area will be

Just wondering what you guys think? Would I be better off installing the "under the seat brackets with the sub frame? Thanks . . .

Thanks guys - I kinda figured that the seat brackets were crucial, which is why when the tech at a local shop told me that they weren't needed didn't make much sense to me.