Hello Mr. Woomer and thank you for your question,
Ribs should not grow together because they are strong bony structures that really don't move within the body and should not come close to each other.
Often, an old injury can result in a bone deformation and cause a lump to develop, however, I would recommend either an ultrasound or an x-ray of that area to make sure it is a bony lump you are feeling and nothing something else.
Most likely there is nothing serious going on but without seeing it and feeling it myself, I would recommend that you check it out.
I hope you find this information helpful. Please feel free to follow-up with any other questions you might have.
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Best wishes and good luck,