Old Eynsford 2F is good. I, like Daryl, have good luck with Swiss 1 1/2 as well.
I'd suggest to try as high as 140 grains Old Eynsford by weight to see if it brings the right/left closer. (maybe still have room for a grease cookie?)

.504" bullets…..If they are shooting individual barrels well enough then the bullets diameter is not a factor. Maybe soft enough to "bump up" if the Woodward .500 BPE groove is a more normal .508"-.510"

Elevation…..eyesight considered, bury the front bead in the crotch bottom of the "V". That is where the sights would have been filed in for to have dead on at 100 yards and maybe 1" high at 50 yards. Level with the top horizontal likely explains the high impacts on target.

Cheers,
Mike

ADDED:
Old Eynsford 2F is certainly a good choice, chance that 3F will push your pressures higher than the original loads.