Major 2,
Just a thought/suggestion...
Check the mainspring length on some of the other, repro Winchester/Henry like firearms you may have on hand.
I have found, in some of my past gun work, on a few occasions, the claw on the end of the spring maybe modified to fit under the stirrup/hammer thro. (the claw or fingers on the mainspring, maybe quite small and still make a good connection)
My best,
Blair