I have used Pauls in the past and I was very happy with the work they did for me. They are very pricey, but as I said they do good work. How expensive? I sent 4 "B" model hub caps that were somewhat beat up to them for a price estimate that came back at $250.00 each. Needless to say I bought new ones from Watts at $125.00 each. http://www.paulschrome.com/
I have also used Royal Silver in Norfolk, VA https://www.rschrome.com/ They did very good work as well.
I have fire truck friends that have used shops in Philly and Ohio and both swear by them, so if those locations are closer to home for you let me know and I'll get the names for you. The shop in Philly can do very large items, so they may be the way to go for the radiator shell.
One of the main reasons it is so expensive to do a fire truck is all of the plating that is involved. I spent over $6K just on plating when I did the 52 and that was not in all that bad condition. I quickly learned that it is much less expensive to buy new stuff it was available instead of repairing and plating the existing items.
One way to lower your cost is to prep the item as much as possible yourself. The quality of plating is dictated by how well it is buffed and polished before it is plated, so if you have the time and motivation to buff the items yourself you will lower your cost considerably since most shops charge $75.00 per hour and up to buff out the pieces.
If your shell is pitted the shop will plate the shell with copper, buff off the copper away from the pits, and keep plating-buffing until the pits are filled and they have a smooth surface to plate.
Pot metal is done with a similar process. A friend sent two "F-350" hood emblems that go on the side of his 56 Ford's hood to Pauls for an estimate.They were pitted and needed the studs repaired as well. Pauls came back at $500.00 each to do them.
Good luck.