209jones,
You might try some "41 mag" pistol bullets for a low recoil load . I used to shoot .41 cal. cast bullets in a .405 win. double. Don't remember the load , but it was pretty close to my regular load with 300 grain bullets and it regulated pretty good.
Let me know if you work up a good "practice load" for your 450/400 . I have a 3" and a 3 1/4" , would love to have a cheap practice load.
Good Luck !!!!
Robert