Still got problems on cold start 454 TBI

[Chevy3+3]

I am the guy that had the backfiring problem on an 88' Chevy Crew w/454 TBI 4x4 Dually only when the overnight low temps were in the 40's or below. Truck has 130k miles and doesnt use any coolant and doesnt burn oil. Since then I have remedied several small issues and changed a few parts but I still have a problem. My fuel mileage is roughly 8-9 MPG normal driving and highway speeds of 55mph. At 55 my engine RPM is about 2400-2500. I have no Service Engine light on after the truck is started.

I believe I am running to rich when the overnight temps get in the 40's. I would get intermittent backfiring for about 20-30 seconds and then the truck would clear up and run smooth the rest of the day. When started the truck idles up to about 1200 RPMs when cold. When warm it idles about 1000 RPMs and about 700 RPM in gear when warm. These RPMs appear to be normal.

Here is a list of what I have done:

1. Replaced the Coolant Temp Sensor
2. Double checked that the EGR valve is working correctly
3. Found and fixed all the vacuum leaks (including replacing TBI baseplate gasket)
4. Checked timing (bypass wire unplugged) and found it was set at 0 BTDC
5. Set initial timing at 4-5 degrees BTDC (bypass wire unplugged)
6. Unhooked battery for a few minutes, plugged bypass wire back in and hooked battery back up.

When I started the truck this morning with a 45 degree outside temp the truck fired up and after about 5 seconds of ignition the truck started popping/backfiring a few times. It did not do it as long as usually. I walked back to the tail pipe and it smelled a VERY strong fuel odor. I almost think it might be running to rich.

I do have a cracked passenger side exhaust manifold. I have a new replacement and plan to install it within the next few days although I do not believe this to be the reason I am backfiring.

I believe something is telling my TBI to dump more fuel causing me to run rich when cold and potentially even when warm. Should I or would it be in my best interest to replace the O2 sensor? There is only one O2 sensor and it is located on the drivers side exhaust manifold and is hooked up with a single wire.

I have also thought about rechecking/resetting my valve clearances or valve preloads

Any thoughts or advice would be greatly appreciated. Thanks