I wasn't able to think of what this could be so I thought I would post it here to see if anyone had suggestions (his responses in black below):

http://www.sweptline.org/forums/viewtopic.php?f=21&t=31506&p=229972#p229972

Starting a new thread since the few-year-old one with the above name was never completed, and I need help on the same issue

Here's a copy of my reply to that thread (posted there so it'd be in context), but here it is - sorry so long, just trying to answer questions ahead of time:

My truck's a '68 D100 w/ a 318 LA and 727 tranny... Has Mallory electronic ignition upgrade installed; she's been my daily driver for over three (four?) years now, she is one of the loves of my life and I am in need of help please!

Wondering if the solution was ever discovered in this ("starting issues - from 2009 I think) thread? I'm having the same problem, - a big click when I turn the key to "start", and that's it... The click is coming from the starter (or the starter solenoid attached to it - haven't been able to get another set of hands to turn the key for me with me under the truck to determine whether the click is the starter or the solenoid that's piggy-backed to the starter). When I jump the contacts with a screwdriver on the starter relay (bolted to the left wheel well), I get the same click as if I were turning the key to "start," just no turning of the starter, and thus no turning over of the engine... As the originator of this thread shared with his problem - it's a good, solid click, seems like something strong is moving - perhaps (?) as if the starter begins to turn and doesn't have enough oomph to turn the engine over (the teeth are meeting and stopping?)...

I had the starter bench tested, and it tested Good. It's a remanufactured Mopar starter I got and installed from NAPA about a year ago... has a 3-year warranty (not that that necessarily means anything, I know)...

I replaced the starter relay for the hell of it today (since it's only $16) - no change - still the same problem - no start, only click...

The battery is good, bench tested it with load - Good. And it's charged (just over 12v when I put my multitester on it). Also double-checked this by trying to jump-start from another vehicle's battery (while the vehicle was running), and the same click occurs - no start.

I've cleaned and ensured good contact on all battery terminal connections, starter connections, starter relay connections. The neutral safety switch prong on the relay is well-grounded - to the transmission. The other prong, which is an orange wire and I believe goes through the firewall and to the ignition switch is making good connection - guessing the ignition switch is not the problem since I get the same click when I jump the contacts with a screwdriver on the starter relay as when I turn the key...

She was starting as normal one day, but the next - just a click upon turning the key, and that way every since. One single click for every time I turn the key to "start." Haven't tried 15-20 times in a row as did the originator of this thread - I'll try that in a few minutes, will update. (*update*: tried 25-30 times in a row, no dice, just sucking juice from the battery with each try).

Stumped. If anyone's still reading this thread, any suggestions will be very much appreciated! This problem is over my head so far... Searching the forums for hours, can't figure it out and this is the first post to match my problem...
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Starting system and starter seem to be doing their thing. Engine/flywheel seem locked up, or just not enough torque being pumped out by starter.. ? The engine has oil in it, I know I didn't run her dry. Gotta get out there and try to turn the drive pulley with a long wrench with the tranny in neutral to see if she'll turn at all... Anyone with any experience/knowledge about possibilities here?
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Holy $*#! I don't recommend being a single dad working full-time plus while going to school... if at all possible.

Finally was able to dedicate some time to my currently non-breathing truck - thank you for the suggestions (also in the engine/driveline section - wasn't sure where my problem was/is stemming from).

Coolant - clean, no signs of oil present; I drained it and had more in the system than I feared - I don't think I was running hot.

Oil - clean, no signs of coolant present, so I don't think I have a crack in the block anywhere to lead to cross-contamination between oil and coolant =

I can turn the engine by hand, so it's not seized - another or

I replaced the starter relay - same response (the single click when I turn the key to start) - so ruled out the starter relay and the ignition switch (same click when I jump the contacts on the relay with a screwdriver).

So I think it may be (as was suggested to the original poster) the solenoid that has gone bad... Any opinions on this? I had it bench tested (the starter/solenoid unit - the solenoid is attached to the side of the starter) and it tested good - possible it would test good and not be?!? Good heavens I hope it's only that.
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I'll post your suggestions over there and see where this goes if anyone has time to respond. Thanks!

. The eng did not lock up all of a sudden & it does turn free, batt is charged & has been jumped, cables/terminals have been cleaned, starter/solenoid did test good if I read that right but if I jumped across the starter & solenoid terminals AT THE STARTER & just a pronounced click & if you're sure the batt posts/large cables are cleaned & I believe they are I'd R&R the starter

I agree tested good does not mean good.
But one more test to try.
turn on the head lights and try to jump the relay, or turn the key.

see if the headlights stay bright, or dim out.

if they dim out, try replacing the ground cable.
I don't know why, but over the years, I have had 2 or 3 die on me.
nothing else wrong with the car, just ground cable no longer carried enough ground.

Bench testing means very little.Ive had starters that would spin great on the bench but wouldnt turn the car over.As said check the cables as they corrode inside where you cant see it.But sounds like the starter is bad.If its the OE they burn the big bolt the Pos cable attaches to and wont start.Rocky

Thanks for taking the time to read guys, I posted your responses over there.