Hi John, I could only find one William Preston enlisted in Alabama (but I don't have all of the resources that the webmasters and others here have). Anyway, this William Preston enlisted as a Private in Co. B, 2nd Battalion, Light Artillery. The muster cards state that he died in Fairgrounds, Hospital in Knoxville, TN in 1862. However, the last few cards have him as William H. Preston, so this may not be your man. Are you sure he served from Alabama?
Allen Davis