I have to admit I went nuts on the chassis before it was blasted. I've a hammer with a long single pick, bit like an ice pick, and I went over the chassis with it, hammering hard, to find any soft bits. Even then the blasting found one or two areas that need extra work.
But, what came back from the galv plant was the same as what went in, except shinier.
I had the chassis of my 73 GT6 galvanised about 12 years ago at MacArthur's in Bristol (now sadly gone) and it was worth every penny of the £47 it cost me.
I've since used Newport Galvanising and found them both expensive and not particularly thorough.
I have also heard good things about Cardiff though and will no doubt give them a try in the future.
As has been mentioned elsewhere, the various dips will take care of the corrosion as long as they can get to them, and make sure every cavity has enough drain holes to let the zinc out!