Engine will not start on cold mornings?

Hello, with an RPM that jumps up that high, it sounds like you have a vacuum leak. That would also explain the hard starting on cold days. An engine takes a larger amount of fuel to start and run when it is cold. This is because colder air is more dense. So, more fuel is needed. With a vacuum leak and more air getting into the engine, it will take a much higher amount of fuel to start. Here is a guide to help you find any vacuum leaks. Being that it is a Ford, I would start with the Intake manifold gasket. It might be running better when it's warm because things are expanding with heat, some gasket leaks will actually seal up on warm engines and leak again when the engine cools down overnight. On intake gasket leaks, most of the time I can find a leak just using a spray bottle and water. You will hear the vacuum leak sucking in the water pretty loud. but do this when it first started and still cold. Also check for any cracked hoses or loose hose connections.
Also, with the age of the vehicle, the fuel injectors may be somewhat clogged up by this point. You can try some dry gas and a fuel treatment to help clear up the injectors, this will help with the computer's fuel control. Techron makes a great fuel treatment cleaner.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Replaced all vacuum lines save 1. The yellow line runs to the back of the engine and when I was replacing, it was not plugged into anything so I couldn’t find anything to attach the hose to.
The motor was redone before I purchased it so I'm not sure if they removed something that should have remained.
Woke up this morning and it wouldn’t start again like it was not getting enough fuel. Cranked fine yesterday afternoon.

On the older vehicles they use engine vacuum to move the heater actuator doors in the dash, so for example, I have an older Ford I kept and it has a small round vacuum tank, its plastic but it has vacuum lines running to it from the intake manifold so that the heater doors can be actuated when the vehicle is off. Do you have any vacuum lines that are not attached to anything and are open to the atmosphere? The high idle is what points to a vacuum leak. Or you may have an idle control valve that is sticking or not functioning correctly. Below are the vacuum line routing diagrams just in case you need them, the 1st one is for the heater system, the other 3 are the different setups possibly for this vehicle.
When it doesn't start in the morning, take out a couple of spark plugs and see if they are wet with fuel or not. If they're not wet then you'll know it's a fueling issue, if they are then it can be a weak ignition coil or bad cap and rotor. If it is trying to start but tends to sputter out, that sounds like a fueling issue, and with the vehicles age, the fuel injectors can be clogged up by now. That will happen with any vehicle eventually. Also, if there is moisture in the gas tank. Some dry gas and fuel treatment will help. But check to see if the spark plugs are fouled out with fuel or not.

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

Problems worsened to not running at all now. Replaced vacuum lines and FPV. It turns over and goes to 500-1,000 RPMs for a brief second and then dies. I could get it to sputter if I continued to pump the gas pedal but never revs up. Today is the first time that it has refused to run at all.

You'll have to put a fuel pressure gauge on it to see if it is losing pressure or if it's something else. Fords have a small port on the fuel rail for a gauge to be hooked up. With the key in the "on" position there should be a 2 second fuel pump prime. It's a matter of testing the basics first to see if there is low fuel pressure or weak ignition spark to start with. Vehicles act up much more when the weather starts to get colder out. Did you check the spark plugs to see if they were wet with fuel or not? Or if there was a lack of spark?

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

https://www.2carpros.com/articles/how-to-test-an-ignition-system

I just pulled the front two spark plugs on each side. They are not wet but man they are black. When I pulled them out the other day, I cleaned them all well and replaced the ones that had issues. Do you know of anything that would be causing them to be so covered in black?

Yes, if the Catalytic Converter has failed. When the exhaust becomes restricted or partially restricted that combustion has no place to go, and carbon builds up on the plugs and in the intake manifold. I wonder if you pulled one of the front oxygen sensors it will be covered in soot too. You can also try starting it with the front oxygen sensor out, it will allow the exhaust pressure a place to go if it's a restricted exhaust. You can also do a back pressure test at the front oxygen sensor hole; most vacuum gauges will also go up to positive 10psi and can be used to do a back pressure test on the Converter. If there's more than 1 or 2 PSI of positive pressure, the exhaust is restricted.
Most times when I check a Converter, I will use a bore scope and actually take a visual look at the converter through the oxygen sensor hole. But a back pressure test will verify the same thing.
If the engine has been running rich for a while, the converters will eventually melt down and block off the exhaust.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms