Before you narrow the spring have you backed out the tensioning screw first? If there is one on your rifle?

And never file across the spring because the scratches will commence it to break.
When narrowing the spring only file the edges along the length of the spring to prevent it from failing.
Or obtain a pistol grip spring from CPA  Rifles instead of trying to use the old straight stock spring.

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Good advice from these guys!  Since things have been reworked also make sure the hammer is not dragging/contacting the stirrup or the spring as it is being cocked. Here is a picture of an original just in case you are wondering what it should look like. The dark one is the original.

Here is an original 44 1/2.  The spring in this action is the same one I showed above.  Not sure if you can grind yours and make it work.

As Dave said...it appears yours could be contacting the hammer as it is cocked or might be trying to push the spring rather than pulling it down because of the angle the spring is at in relation to the stirrup.  A proper spring from CPA might be worth a try without the spacer.  If you can reharden and temper a spring you could possibly reshape it and get it working.

Now that I look closer your spring appears to be attached at sear, set trigger springs point which would put it way to close to the hammer.  You can see the sear spring in my photo.
It also appears your tang starts to bend much closer to the frame than mine.  If this is the case not sure the CPA spring will work.

The top tang also looks to be about half the length of the original so I assume the lower is also short.
 
If I'm correct about this, a spring like shown in this photo may work, but would need to be a custom spring.  This is not a 44 1/2 but a #7 rolling block but you get the idea.

oneatatime, the more I look at your photo the more I’m surprised you can even cock the hammer with that angle on the spring.  The geometry just looks way off and I imagine it could be putting lots of stress on the stirrup when cocking.  You probably don’t want to hear this, but I think to fix it, the spring needs to be moved back down the lower tang if there’s room and make a new spring that attacks the hammer stirrup at much less of an angle, I think as straight in as possible is best.  It also looks like the screw has had a vice grip on it and that’s because the screw is under the top tang making it hard to use a screwdriver.  Moving the spring  back will fix that issue also.  But then I may be all wet about my assumptions!
Bob

Thank you all for your observations and suggestions. In light of them I examined the possibilities. There is no other place to mount the spring. There is no tensioning screw. The spring could be narrowed some so I looked at the screw attaching it. It was no sweat to used a screwdriver bit and a ratchet handle. It must have been attacked with pliers in its past. Surprisingly after a few clicks on the ratchet the tension was really reduced on the hammer. By retightening and checking the hammer bit by bit a hammer tension like my 44 Stevens has was reached and it was actually only a hair off from full tight. This was the tension I remembered when I was shooting it as a 219 Donaldson 3 years ago. My gunsmith must have snugged it up not realizing that besides the retaining screw it was also serving as the tensioning screw. So all is well and the new BRC choked 32-20 barrel is on it and ready for a trip to the range! Thank you all.

You should never narrow a spring. The correct way to reduce the rate is to reduce the thickness. Spring steel can be filed and then polished without too much effort. The local stresses in a spring are a function of the thickness: by narrowing the spring you do nothing for its fatigue life, but by reducing the thickness the fatigue life is inproved.
Fred