Engine won't start or turn over...1965 Chev w/350 engine

[Old Blue 1965]

I went to start my truck 1965 C-10 with a 1971 350 motor after it had been running. I had just replaced my heater core and bolted it in and checked for leaks and topped it off with antifreeze.
I then found a leak from the fuel pump, turned it off. Replaced the fuel pump and went to click the starter to move the cam, it clicked, and on the next try NOTHING. No starter click just the lights dim when turning the key. It is as if the battery was dead, charged it overnight and still nothing?????

Any ideas where I should start to figure out what could have happened. Maybe starter, ignition switch? I just had installed a Pertronics electronic ignition to replace points, also coil, wires, cap & rotor and plugs. Engine ran really great, like a new motor.

Please help me to figure out where to begin the diagnosis of this problem.

Thank you very much, Wongeroo guided me thru the heater core R&R and was perfect!

Craig