Speer shows 2300 fps at 76 grains with the 350 grain bullet. No data for the 400 but you could compare it to the 45-70 loads for a cross reference.

Looking at Hodgedon data: https://www.hodgdonreloading.com/data/rifle a WAG number of about 68-70 grains will get you into the 450/400 or 404 Jeffrey range. Since this is below the starting load you could just begin at 68 grains and work up to see if you can get good accuracy.

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