The Smith Shop 1873 Lever Safety Bar Spring
First, remove the trigger group from the rifle and remove the mainspring.
Then remove the current flat Safety Spring. Re-install the screw that holds the trigger spring in place. Sometimes it will protrude thru the bottom of the tang. You can take some of the meat off so it is flush, or add a small washer to take the place of the removed flat spring.
Next, tap out the cross pin that holds the safety bar.
See picture #1 below.
Set the new coil spring in place to see if there is enough width for it to fit.
See picture #2 below.
If so, proceed to install. If not, use a file to remove a little material (usually no more than .020" is needed) from the side of the safety finger where the spring will sit.
See picture #3 below where the pointer is pointing.
Install the new coil spring per picture #4 and use a pin punch to keep it in line.
See picture #4 below.
Tap the cross pin back into place.
Picture #5 below.
Check to see that it works and the leg of the spring engages enough of the safety. If not, bend it in as necessary.
Picture #6 below.