Hi Steve,
Not really anything you can do to modify your existing suspension spring. The first thing I would probably check is to see if it does fit in the case with the correct pendulum length. Just in case things are actually ok.
If that doesn’t work and you haven’t already tried you might check that the locking and drop on BOTH entry and exit pallets is deep enough and equal. If the lock is too shallow you may have an escape tooth skipping by without receiving impulse. This will cause a clock to run fast. If I remember correctly your clock had a half dead beat escapement so sufficient lock will be important.
Please let us know what you come up with Steve,
Bob