If I had a garage and if I had this problem I'd make a portable spring testing jig to take to the store.
Then, I'd preload the candidate spring 1/4th of a turn and then measure the torque to get to the next 1/4th turn, so by now the spring is twisted 1/2 turn. Subtract the two lb-ft values and scale up from there.
The Web is full of advice on this subject and one link had a calculator that works for their spring inventory and a 4" drum diameter.
A drum with more than one diameter is little messier to figure.
The more numbers you have, the more measurements you take, the sooner you will know if your current strategy for this project will likely work out.