ClassicBallistix offers an aftermarket cylinder for the Ruger that adds about 10 grns of volume. Still short of the Walker's capacity, but certainly gives a Ruger more oomph.
With a more energetic powder (Swiss, Olde Eynsford, or Triple 7) you can easily reach or surpass 500 ft/lbs of energy with a conical from a Ruger. Mike Beliveau did some testing with T7 vs Goex and RB's and 2 conicals, and though he reduced the loads ~15% and lightly compressed the T7, was able to achieve near 500 ft/lbs of energy, though the reduction of T7 is stated such not for safety but to achieve similar results as standard BP loadings, and a Ruger has no need of reducing anyway as it isn't built with mild steel.
Supposedly people report roughly 700 ft/lbs with the ClassicBallistic cylinders, though I'd think that a little on the high side.