Weak percussion springs cause excessive hammer bounce, gas escape and this can result in fragments, etc, hitting the shooter in the face. So long as there is a cap on the nipple a stiff mainspring should not cause any problems. Personally, if it were me I would cut the barrel back and use “patent” type percussion breech as is found on the spuriously stamped heavy long rifle conversion with the Hawken stamp on the barrel. While its possible Hawken may have converted it and even stamped the barrel they did not make that rifle.
W. Greener in the “The Gun” from 1835 was not a fan of converting flintlocks to percussion with a drum and nipple and I am not either.