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Searning "74F258" brings up this item. Should've linked it in my last post, my apologies.

Yes, the 258s form the address mux for the RAM. If any of them fail it absolutely will cause death chimes.

Yep, both appear to be in stock at Mouser. They're pretty bog-standard parts.

If you think there's a chance you might've baked the chips with the heat gun, I'd honestly just go for replacing them. Those parts are still available and cost pennies, would potentially save you the trouble of having to desolder the old ones yet again

One thing I noticed in your fourth photo is that there appears to be some leftover solder paste that didn't reflow properly, would be a good idea to clean that up. The excess solder on some joints could've easily bridged to nearby tracks, solder wick + flux will help tremendously there as others...

Some PSUs will make a ticking sound when not connected to a load, the fact that it ticks when connected to the motherboard could be the fault of either the motherboard or the PSU. Inspect the replaced caps on the motherboard to make absolutely sure that none of their solder joints are shorted to...

A damaged shadow mask or yoke are unlikely to be the culprits here; the former would generally cause purity issues, as for the latter, the geometry/width/height all seem to be about what they should be. Go over your work like the others have said in case of any loose connections or bad solder...

Decades' worth of Apple service info, forum postings, etc. all strongly advise that the CRT be discharged to the ground lug on the mounting ear, else the rest of the system gets damaged. Person with the dead 128k doesn't specify how he did it, entirely possible the damage is down to user error...

Assuming you've checked the easy things, e.g. caps, rotten traces, whether or not you get sound from the headphone jack... Many of these LC-derived designs use a 78L08 for the sound IC's (DFAC) analog supply and it's prone to being attacked by cap goo. It should have +12V on one side and +8V on...

If you actually have a hot air rework station, the risk you run using it over dual soldering irons is considerably lower. Emphasis on the "rework station" part; the kind of heat gun you find at the hardware store isn't really optimal for this kind of work. Heating and lifting each side of the...

"Twist and Push" method is just grabbing the capacitor can with a pair of needle-nose pliers, pushing down, and twisting it a little bit back and forth until the leads inside the can fatigue and break. If the plastic base of the cap is left behind, remove it with tweezers. You'll also have to...

Grasping at straws here: In your recapping thread for this card, you posted a picture of the card after corrosion was removed w/ vinegar. That picture showed the component labeled SOMC1603, a resistor pack, and it appears that several signals going to the floppy port go through that component...

If it still boots I highly doubt you've messed up the board. Pictures of the board will help if you want to verify everything is soldered right.

If the traces for the negative sides aren't completely gone, follow them back until you find somewhere to solder a jumper wire. Otherwise, look for the schematics and use them to find a place for the wire.

As far as logic boards are concerned I'm pretty sure it doesn't matter. I've used both types on boards of mine and have never had problems. Looks like corrosion, vinegar may help. Best not to leave it on there.

The chances of you finding a bad cap connection on the logic board are slim to none since the 650 uses tantalum caps. Like @jeremywork said, the green hue is almost certainly because the Centris is outputting SoG (Sync-on-green), so you need to find a monitor/adapter combo that doesn't have this...

If the fuse was blown it wouldn't even be able to intermittently recognize the keyboard/mouse. More likely is that the capacitors on your logic board need to be replaced, especially if the Classic's making whistling noises. There are a few between the ADB port and the MCU for the keyboard/mouse...

To rule out the obvious: have you tried swapping the IWM with a known good chip, is the drive ribbon cable known to be good, and is the disk used known to be good? Very bizarre problem for sure, I'll get my good SE 800k board and take some measurements to compare against your boards.

There was similar stuff on the underside of the board in my LC II when I got it. No idea what it is, perhaps something leftover from manufacturing, but thankfully it seems to be harmless.  In the meantime, it would still be a good idea to get rid of the caps and clean up the board to stop the...

As far as I can tell, your P450 description sounds normal. Nothing is shorting VCC straight to ground. I'm still suspicious of the power supply; try taking out everything unnecessary but leave the FPU in and see if that changes anything, e.g. remove RAM/VRAM SIMMs, SCSI2SD, keyboard, etc.