It is already cooked.
Not sure about weight anymore...
2 gallon water
3.5 cups salt
8 teaspoons Salt #1
6 tablespoons pickling spices
(Small print: reduce salt to 2 cup if using Morton's Kosher)
Hi there and welcome!
Even though its cooked you can still soak a corned beef to draw out the salt. I would give it a try for sure.
Now I see the issue why it is too salty and it is a common one.
Most recipes online for stuff like brining and curing give a generalization of how much salt/seasoning to use but dont address the "science" of it that nails it every time.
A sure fire way to nail this every time is to do the following anytime you brine or cure (brine+ cure#1 salt).
Add up the weight of your meat in grams or ounces (grams is for easier math).
Add up the weight of your water in grams or ounces (1gallon of water is a little over 8 pounds = 8 x16 for ounces or around 3,632 gm).
Now take the total weight which is water weight + meat weight.
Multiply that total weight by 0.0165 (this is 1.65%) to get the weight of the salt you need to use.
If you do this you will NEVER over salt your meat again no matter how long it brines.
This is what is called an equilibrium brine.
I also highly recommend you dissolve the salt and liquid together and then inject it into the meat all over and let it sit. This will drastically speed up the brine time in big cuts of meat like turkeys, whole chickens, etc. because the brine is working from inside out as well as outside in.
Doing this math for a brine eliminates the possibility of ever being too salty or too bland. You nail it every time and you know why where following a random recipe that doesn't tackle the problem like this will be hit or miss on your results and definitely not consistently repeatable... as you have found out :)
I hope this info helps :)