Hey everyone, looking for some help please.

Here's a quick rundown on my truck and how it got here. Had it for 10 years, kept up on maintenance, been running like a champ until last year. Was on the highway and my truck simply died. I figured it was an electrical problem but the idiot of a mechanic swore my engine had seized and needed to be replaced. A second opinion revealed the same. Bad coincidence. After installing an engine from the junkyard and also a new fuel pump, the truck was still dead. To keep it short, he eventually ordered a new PCM at which point in came to life. Can you say "I told you so!". Hasn't been running same since and I attribute it to a crappy engine install by said mechanic.

Fast forward to now. The check engine light is still on and the girl is running worse. It lacks power. I've poked around under the hood and cleaned up as much as I could. At once it had 8 codes, it now has only 2.

P0174
-Fuel trim bank 2 condition

P1504
-IAC circuit fault

I have found and replaced damaged and clogged hoses, cleaned the TB, replaced IAC, replaced plugs, wires, MAF, bank 2 oxygen sensor 1, and some other things here and there to no avail. Everytime I pull #8 spark plug it has black gunk on it. All the other plugs are fine. Not sure if it matters but I can hear the cat ticking under the driver's side which I've never noticed before.

Suggestions? Fuel pump, misc sensors, leaks, ignition coil, fuel injector, etc? Anything helps. To make matters worse, vehicle registration was due a month ago and obviously it won't pass smog. But I just want her to run good again! Thanks in advance!

PS: Pretty sure this isn't related but the tranny is shot and will need to be replaced once all this is fixed. I'm not too familiar with transmission problems but it seems like it's running in one gear. It won't downshift and I need to keep it in 2nd so it's driveable. Main priority is above though.