Electrical? Coil? Problem Need Help With
 

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?

On the engines without HEI, there are two circuits for power to the coil. One from the ignition switch to the ballast resistor, then to the coil. The other is from the solenoid on the starter directly to the coil, bypassing the ballast resistor and providing more voltage to the coil, making starting easier. Sounds like your first circuit isn't working, I would guess something got disconnected, possibly a bad resistor or ignition switch problems.

Look for the resistor on the firewall. I should be ceramic and roughly 4" long with two wires.

Sounds like your 12v wire to your coil has a bad connection. You have a 12v volt wire from the starter to the coil to provide power during starting and one from inside the cab, that is a resistance wire, that powers the coil during running. Check out your coil to see if it has two wires on the + side. Chase them back till you find the one that goes to the cab. follow it inside to make sure it is not disconnected from the fuse box. Also, check to make sure there is not a loose wire on the back of the ignition switch.
As a temporary measure run a jumper wire from the battery to the + side of the coil and start the motor.