I'd knock off any big chucks of grime and then have the block dipped. The machine shop you use may be able to do it for you.
Last time I rebuilt a SBC the Intertubes weren't around so I'm not familiar with any web sites. Googling "
SBC rebuild" turns up a lot of interesting stuff. Most auto parts stores should still have rebuild books for SBCs.
Before you invest too much in this project, you really need to get the block inspected by a machine shop. It may not be rebuildable.