Running lean no acceleration?

Hello, if the exhaust manifold is glowing red hot, you have an exhaust restriction, i.E. The Catalytic Converter. You can either drop the front of the cat to allow the exhaust to escape on a quick test drive or do a back pressure test at one of the front oxygen sensor locations. If you need more information on back pressure testing let us know, but you're going to find the catalytic converter melted down and clogging up the exhaust. I wouldn't drive it like that any longer until you can get a look at the cat from either the front or back, whichever is easier. You will end up melting other components if you keep driving it like this. I'll take a look at the exhaust system diagrams and post what's there.

This is a diagram of the exhaust system, with the catalytic converter marked out. I have seen vehicles flying down the highway with the cat glowing red, and the vehicle on fire a couple miles down the road. When they get that bad, it will start to melt EGR components, and anything else made of plastic, along with warp the manifolds. I like these 4-cylinder rangers, much easier to work on than the V6, I can see it has pretty high mileage already, but if taken care of they will keep on going. We have a 01 4.0L V6 Ranger and the timing chains are a nightmare to do.
There isn't a check engine light on with a P0420 code set?

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

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

It sounds like some of the old cat got stuck somewhere further down the pipe. That happens a lot when they melt and break apart. Pieces get stuck in the rest of the exhaust. But a back pressure test a the front oxygen sensor location will verify if it's a restricted exhaust or not without having to unbolt the exhaust, You just need an adapter to screw into the oxygen sensor location and a vacuum gauge that can read up to 10psi, Harbor Freight sells on that reads very low psi. Just don't leave the pressure gauge on there long or it will melt, you just need to get a quick reading at idle and have someone rev it really quick and see how high the back pressure goes. It shouldn't be more than 1-2psi max. Thats where I would start.
Do you have a scan tool that can read live engine data? And the engine is not overheating at all, it would take a lot for the manifold to glow, that's some serious heat buildup.
If you have access to engine data on this, which you might be able to read OBD2 data on this being a 95', but the engine data should show you if it's actually a lean condition through the Long Term and Short-Term Fuel Trims. They didn't take out the front oxygen sensor too, did they? I assume you have pretty easy emissions standards where you live.

But I guess it can't get stuck if there's no original pipe left, I would plug the oxygen sensor back in and see if you can get a reading off of it. Or even with a multimeter to see if the voltage is stuck low. Anything below 0.450 V is considered to be a lean reading. Above is rich.

Do you have a fuel pressure gauge and an injector pulse tool? Both are fairly cheap, and you can do an injector balance test in just a few minutes to see which ones are flowing more or less. I can link some tools if you want to do this test. The injector pulse tool is about $30.

Okay, so if at idle, your Short-Term is +5.5% for example and the Long Term is at -10%, (make sure to note whether its positive or negative), in this case you add the two together and the total trim is -4.5% which isn't bad. If you get a high negative number (i.E. -15%) then the PCM is compensating for what it sees as a rich engine, and it is reducing its fuel strategy by 15% to bring the air/fuel mixture back into emissions standards. This applies differently at idle and higher rpms (2500), a vacuum leak will show itself in the fuel trims at idle with high positive numbers (+15%) for example, and the fuel trims will come back down towards 0 when you raise the rpm. You want your fuel trims to be as close to zero as possible, at least under 10% +/- is the max I like to see.

Even there something is going on, but if you have leaking injectors the PCM will reduce the amount of fuel being delivered to compensate for the extra fuel being leaked by the injectors. Even with a fuel pressure gauge on the fuel rail, you should see it hold pressure when the key is turned off, pressure shouldn't drop off quickly.

Is the front 02 staying more low than high? If you can graph the 02-voltage reading you will be able to see it better, if its hanging/trending more lean (low voltage) that high, 0.620 is a little low. It should fluctuate between 0.2-0.8v in a perfect world, but this were checking the fuel trims at idle and 2500rpm will come in. Because certain conditions will change the fuel trims.
If it is running that lean and adding so much fuel that its overheating the manifold, that a lot of fuel being added, it might be that the PCM is getting bad reading from the MAF and overcompensating when it doesn't need to. I know the test port on the fuel rail can be a pain sometimes when it's in a tight spot. But that will tell you if the injectors are leaking.