Good Luck with your car. The thing about kit cars is you often have to come up with innovative solutions when it comes to repairs.
I am not that familiar with the bulldog. At the end of the day the fuel tank is just container to hold fuel, and there are a number of fabricators of custom fuel tanks if you search the internet. It might be a good idea to go along to Stoneleigh Kit car show, which is coming up soon, and talk with some of the companies there, take some measurements and photos.
If the tank a relatively simple rectangular box it is likely a custom tank made by or for Pilgrim, or if it has a more complex shape it may have come from the donor? Do you know if your Bulldog is based on the Morris Marina, I think some later were based on Ford running gear.