Transom on my 170 is solid too. It appears to be the original.
I really don't relish the idea of drilling holes anywhere in a hull but the safety factor in having a pump help protect the whole investment outweighs my prejudice. I agree with Wilson; if you start filling with water, be it from rain, seas or hull damage, every drop you can get out of the hull may make the difference.
The law of "How Things Work Out" dictates that if you put the pump in you'll never need it. If you don't put it in, you surely will need it.