1999 GMC Yukon Need some help BAD!

Engine Mechanical problem
1999 GMC Yukon V8 Two Wheel Drive 170000 miles

I have a 99 yukon with the 5.7 vortec. I bought it for 1500 dollars. Started the yuk, it ran alright. Got onto the highway, nailed the gas and it just bogged on me. I was able to do 60 at this time. Then as it heated up, it got slower, down to 10 (in a 65) and couldnt even push the gas pedal 1/8 of the way down before it bogged out. So I changed the fuel filter. It wasnt the problem. Talked to a few mechanics n friends n they told me that the fuel pump in the yukons suck a**. So I changed the fuel pump. And that was the problem. Started it, nailed the gas pedal a few times and it roared like a champ. It was already about 9 pm, so I just covered everything up and went home. I didnt realize how big the tank was, it was half full when we changed it, and I couldnt get it back up. I tried to syphon, pump, but couldnt get the gas out, so I said f it, ill just start the yuk and let it burn the gas like that. Well, I turned the key, and the motor wouldnt start. It would turn over, but wouldnt start, and when I turn the key off after tryna start it, it would start diesiling, blowing back up. So I was like **** ***** *** *** ***** ****. I just couldnt understand, it was runnin like a champion the other day. Checked the spark, its sparking. Hit the fuel release valve, ppsssshhhhhhh, it scared me to see so much gas comin out lol, I could just imagine what it was gonna cost me to drive around. So its gettin gas, gettin spark, I dont think its the cat's because this just happend overnight. Now, I was told it could be my timing chain, or the cam itself. I was also told it could be the crankshaft position sensor, I was also told it could be the coil pack. If yall would just give me some pointers as to what to do, and the process of elimination, and what yall would check first. I really need help, I always wanted a yukon, finally got one, and it dont run. THANKS A MILLION