Hello everybody, I'm a newbie on here. I'm on here to learn more about the tbi system. Mainly how to get my 62 suburban driving down the road reliably. Heres the story in short. I bought a 1962 gmc suburban took out the 305 6cyl manual tranny. For a 1989 chevy 350tbi that I got from a guy that owned a shop. I was told the tranny was a 700r4 which in later I found out was a th400. Which is fine I guess. It had no computer or wiring harness so I got a ecm from a wrecking yard its a # 1228747. Which I think would work for it. (I'm a beginner on this stuff) I bought and wired up a painless tbi harness.

So my truck idles pretty good sounds great. Having problems getting it to drive great. When put in gear and try to actually drive the thing its sluggish and backfires which sounds like in the engine compartment. I've set timing by first unplugging that cigar shaped plug by the ecm and set to 0 let run turned off and plugged back in. First time I did this I drove it and it ran great around the neighborhood. I was excited! Next day I got in and it was back to wjere it was. Looked at timing with my timing light. It was back to where it was before I timed it. Which made it run crappy.
What I'm I doing wrong? Any advice would help out a lot. One more thing is my motor at least the gauge says it running hot it gets to 240 than I turn it off. I have a new 190 degrre thermo in it. The heater blows warm-hot air. Clogged radiator? I purged the system as well thinking it could of been a air bubble.
Basically any help with any knowledgable advice would be hugely appreciated!! Thanks a lot for reading this long intro.