Recently bought a 68 Firebird with the current SBC and water pump in it (see photo). Having a hard time figuring out which alternator to use, where on the block to mount it, and which brackets to use to mount it correctly. Please help!

Thanks

 

The length of pump will determine brackets. Check distance between pump and timing cover.
If no space, then use '68 & earlier bracketry - short water pump, as this set-up will match wire harness from the body, hopefully.
If a space between pump and timing cover, then use '69 & later bracketry - long water pump, and extend wires to hook up.
Also, pulleys will be needed in driving accessories.