Thanks for the info. Interestingly, I put a hardwood shim under the front of this gun already. I have a Ruger No. 3 in 30-40 Krag. With full loads and the steel buttplate, it gave me black and blue marks. It also strung vertically. I read Frank de Haas and fabricated three fitted hardwood shims of different thickness (also rounded top and bottom to fit the barrel and mainspring bar). I ended up with the 1/32" over size. Similar to what you and de Haas came up with. It stopped the vertical stringing. Anyway, I am fitting a new No. 1 buttstock to it now, but don't plan to change the forearm.
I bought a Ruger No. 3 wildcat recently. It is a 7mm/30-40 Krag Improved. Just about identical to the .280 Remington. It has a custom No. 1 stock on it. Recoil is a pussycat. That convinced me to change the other one.