If you don't have the correct alt you can't just hook it up with one wire to the batt. This is where the test meter will help determine if your alt is creating an output. But first, you need to figure out why the engine won't turn over.Don't have another top post but it is on a charger right now, I have looked at and watched conversions to 1 wire alt hookups for Chevys. can't help but think this one as done wrong. not shows a wire going from the alt straight to the battery.
What do the headlights do when you turn them on?
Get us some pictures of what you have for an alt, external regulator and wiring. From there I or we can help with a plan.
All the power for the ignition and headlights comes in through a black wire running from the horn relay through a fusible link then the wire turns orange and feeds the rest of the cab. The fusible link may be bad. You need to see if you have power at the orange wire near the fusible link. If so, check for power at the fuse box.
I'll be back in the am. gotta go.