1980 C20 Died

[44SAA]

So I was on the highway admiring the scenery when all of a sudden I lost all power. Thankfully, I was in a safe spot and pulled over to the shoulder with no issues. Hooked up a jump pack- no start. Got a tow safely home. My alternator is OEM so I figured it gave up the ghost and drained the bat. Purchased a new alt, charged the bat and success. However, today while attempting to start no joy. Cleaned and checked all the grounds. Still have spark. I’m at a loss. When I turn the key interior lights and fan blows. It’s almost like I triggered a kill switch. I know this is vague and I’m barely handy with a wrench, but looking for some solicited advice.

Thanks 44-40