Electrical? Coil? Problem Need Help With

[Grich]

I purchased a 69 C-10 a few weeks ago. I pulled the rest of the A/c unit including the Heater-core, air ducts and hoses. It did not have a compressor. When going to move it at the end of the day, it will start and run for a few seconds until I let off of the key back to "Ignition On". I pulled the alternator and replaced it, followed all the main wires and checked for power and all seem ok. When I purchased the vehicle it ran fine. I drove it for a week daily. Prior to me buying, it sat for 2 years and only started twice.
I want to continue working on it but have to get it started to move into the garage......

Any idea what the problem could be?