Springs for Dummies (me):
There are two kinds of springs; compression (those that resist being compressed) and tension (those that resist being stretched). Springs are rated something along the lines of how many pounds or ounces it will support or hold before it starts to compress or stretch.
The actual formula is more complex than that, taking into account constant rate or variable rate and other factors like material and design; belleville, bi-metal, wire coil, etc.
Take your old spring along with you and see if any "feel" like they're about the same stiffness. Look for ones with at least as many coils as your old one. A clutch return spring may be stiff enough but they usually have only a dozen or so coils, They may not have enough travel and will deform or break after a few times.
When it comes time to install; you'll soon see that getting it stretched out over the ears/pins/eyelet holes is a real chore-- and you may also learn first-hand just how far you get get one to fly or how many glass panes/mirrors/canning jars you can break if it gets away from you.
Here's a neat trick that'll take the hazard/danger/fun out of wrestling the spring into it's place.
First, measure the distance from hook to hook to see how long the spring needs to be when it's installed without the brake pedal being pushed down and add a 1/4" to 3/8" to the measurement.
Next, drop a bolt down through the hook end of the spring and grab the bolt in a vice (the head of the bolt will keep the spring from getting away if you lose your grip on the free end). Have a friend help stretch the spring using another bolt or round bar as a pry in the free end . Grabbing he spring with vice grips or water pump pliers may gouge the spring wire and can lead to stress-risers and early breakage (not good unless you enjoy replacing the little suckers).
Now comes the "Wow, why didn't I think of that" part; With the spring stretched out, stick a postage stamp-sized piece of folded business card or card stock between each open coil. When you or your helper let go of the spring, the pieces of card stock will keep the coils expanded and will give you the length you need to easily get the ends of the spring over the ends of the pedal and spring keeper.
Having already measured, you'll know how long the spring needs to be to get it into place and you can adjust to that length by adding or removing pieces of card stock.
Once the spring is in position, pushing down on the brake pedal should allow the pieces of card stock to fall right out, but I still crawl underneath to make sure all are out.
