"...no stainless steel Ruger GP100..." Assuming you can get .327 ammo and/or brass, look into a Regular SS GP100 in .327 and buy custom grips. Mind you, you really don't need custom grips. Shot a GP100 with a trigger job and nothing else in ISU bullseye matches, the forerunner of ISSF, for eons.
If you want to shoot .32 S&W Long in bullseye competition, you'd really be better with a pistol vs a revolver. Pardini, Morini, Hämmerli, Benelli, and Walther, among others, but chambered for wadcutter bullet type all make 'em. High end expensive kit though.
https://www.carl-walther.com/product...t-pistols.html