mark

i have a couple of those guns as well (neat little gun

) the weight issue will take you into painfull expirences if you try to go down the 577 route, i think the recoil will be to much both for your body and for the wrist area in the gun.
i think a 20 bore rifle sleve job would be the answer and then make it a 2,5-3 drams load and a round ball, you could actually short chamber it, in case you sell it no sad soul will blow themself up by putting a high energy round in it.
just keep the loads on the light side and you have a stalking rifle to die for.

best

peter