Need help for an Alternate Timeline

[Tigerdovefan34]

So I'm beginning the process of writing a TL starting in the 1850s where a bloodier and more decade leads to the North getting fed up with Southern Antics and electing Radical Republican Charles Sumner of Massachusetts as President. Due to Sumner being a major anti-slavery advocate, the four Upper Confederate that seceded OTL after Sumter (AR, NC, TN, and VA) secede with the rest of the CSA as well as the states of KY and MO (MD tried to secede but Scott sent in troops to Baltimore to make sure that didn't happen). Preston A. Brooks is elected President of the CSA as a way of rubbing it in to Sumner's face. Sumner declares this as a war to both end the peculiar/vile institution of Slavery and to reunite the Union (so some foreign generals like Italian hero Garibaldi joins their cause) and calls for 100,000 White and 50,000 Black Volunteers, planning to bludgeon the CSA with one massive army and end the war then and there. Of course, that doesn't happen and the Civil War lasts from 1861-1867 ITTL (Nashville being the 1864 Miracle for Sumner to be re-elected instead of Atlanta as OTL).

I already have the battles and campaigns of the Eastern Theatre worked out but I need help with the Western and Trans-Mississippi theatre of the war. If anyone is interested in helping me, please post here.