What else besides battery or starter?

Possible short to ground.

i'd probably bypass some things and see if starter motor will crank the engine, if you haven't already tried? One way is to jumper load side of starter relay. Pull the relay, use jumper wire between cavities 30 and 87, the starter motor should crank the engine. if you try that make sure the transmission is in park or neutral and parking brake is set. make sure vehicle can't move. terminal 30 is hot all the time and fuse protected. there is also a starter fuse, you can use a test light to check for voltage. at the starter motor, two voltage circuits, one comes from the battery, hot all the time. the other circuit goes hot when relay is energized. the starter motor grounds through engine block, ground is important, also.

Chevy known to have a loose ground. Tow of them are on the firewall.