Joining up to learn about the slant in the 1972 D100 I "accidentally" bought. It's a funny origin story. The truck was donated to my gun club and was used as a shooting position (engaging targets out the windows, bed or crouching down behind the body) a few times. It quit running 10ish years ago and just sat behind a storage building. I was at a meeting to plan a big pistol match this year and there was mention of using the truck again but it didn't run. I said I could probably get it running. Then it was mentioned the windshield would have to come out for the planned use. That hurt my soul because I thought it too nice to destroy like that. I obviously have ZERO poker face because the match director could tell I didn't like the idea and then he asked exactly the wrong thing of a car junkie like me: "well, do YOU want it?". The club needed to figure out what to do with it because we're relocating later this year. So the board voted to "sell" it to me cheap. Cheap enough I'll need a gift letter if I ever get to the DMV to put it back on the road.

I've managed to get it to crank. Had to replace the tank pickup. But the Holley 1 barrel leaks so it'll probably need a rebuild. I moved it in reverse and am unsure that 727 is happy.

Right now it won't crank again. It tuns over a bit and the starter disengages the flywheel and just spins. I probably need a fuel pump too. The current one seems to lose its prime and it takes forever to get fuel back up from the tank. I had to "help" it prime by siphoning through the fuel line when I did get it to start.

I threw some short videos on YouTube.

The first start and run: https://youtu.be/eQMSUBBrPB0

When I got it to idle briefly: https://youtu.be/88xdhvHhzVQ

The current issue with the starter: https://youtu.be/hHH7IYylWHI

And Pics:

I didn't watch the videos but you said "won't crank again" but you say you had it running/ or trying to at least...
When it won't crank will the engine not spin over or will it just not fire? If the engine spins over it is "cranking".
If it spins but won't fire the list of possibilities is different than if it just "clicks" or "won't do anything".

I'm guessing that trans drained it's torque converter back into the pan after sitting that long, and the pump works different in park than neutral, it won't fill the converter in park but will in neutral.... Hopefully the fluid doesn't look like a strawberry milkshake.. (means it got water in it from its time of sitting)

I know where there's a 72 that's been sitting forever, I've actually been wanting to go knock on the door and ask about it.... I hate seeing it just sit. But I don't have a place for it right now and I have a newer (85) truck as my current project already...
I don't know anything about what that 72 has, don't have, what may be wrong with it, etc. kind of how my owning this 85 started out...

Got a question/ what part of the country are you in? People here have parts, know how and more, knowing what area you're in might open up some possibilities there towards being able to get it going again

First, replace the starter. Its drive unit clutch is failing, which is why it'll crank a bit then spin like that.

Then you can figure out what else is wrong.

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Too many people who were born on third base actually believe they've hit a triple.

I didn't state that correctly. Won't start because it's not getting fuel. I fought and fought to get fuel to the carb the first time. Lots of siphon on the fuel line until I saw it squirt disconnected from the carb. Now I've disconnected again and when the starter does work I'm getting nothing out of the line so the pump is probably just week or what little running it did killed it.

Sorry forgot to answer this part. Central Alabama just Southeast of Birmingham.

For some reason I see this more on Dodge trucks than Dodge cars but I've seen several fuel sending units either rot off about halfway up or else they get pinholes in them...
Ever try to suck a drink thru a hole in the straw?

Already had to replace the pickup. It was plugged up solid. I was able to blow air through the rest of the lines. I did replace all the rubber lines. I could blow through the filter so I left it but have a new one on the way.

Set your 5 gallon lawnmower gas can next to the truck and string a hose from it to the inlet of the fuel pump to get it running on for now

Fuel pump lobe wear on the cam will make the pump seem bad. May need to add an electric pump.

ETA Nice truck!

Forgot to update this a few days ago. I got it to run long enough to drive around the yard a little. Replaced the fuel pump and it fired up and ran so-so but as I was putting tools away to try moving it I saw a puddle of gas underneath. Turned out the steel line from under the cab along the frame had a pinhole rusted in it. Once I fixed that it fired right up and idled.

Now I'm waiting on my brake booster to come back from Cardone and meanwhile I need to tackle the cooling system.