I'm new here, but no where near new to cars and trucks.

Name is Jason.
ASE Certified mechanic
Over 15 years experience.

A few months ago I bought a 1999 GMC Suburban 1500 4x4 with a 5.7L

I bought the truck knowing it had a misfire.
Once I got the truck in my shop, I found that it was a cylinder 3 misfire and that it was burning coolant.
I assumed that it was a head gasket since after it warmed up the misfire went away and it ran great.
After it warmed up and was running good, everything on my SnapOn Modis scanner looked perfect including the fuel trims.
Recently I went in to replace the head gaskets only to find that both heads were cracked, and on the left head the crack went into cylinder 3 near the exhaust port.
So I replaced the heads with remans and threw it back together.
Since then, I've had high negative fuel trims but only under certain circumstances.
When I first start it up in the morning, the fuel trims range from -6 to -12.
Once the engine warms up, the fuel trims go to normal. (+ to - 2)
While I'm driving and my foot is on the gas, the fuel trims are normal... But when I take my foot off the gas to coast or come to a stop, the fuel trims jump back to -6 to -12 but run normal after a few seconds.
For the most part I wouldn't worry too much, but when I step on the gas a lot of white smoke comes out of the exhaust. As far as I can tell it doesn't smell like coolant, but it does have a rich fuel smell.

So far the tests I've done are as follows...
Smoke tested the intake system for vacuum leaks. No Leaks.
Tested the fuel pressure by keying on, then off and checking for leak back. Holds fuel pressure after key-off.
Smoke tested EVAP system. Tested good.
Replaced PCV Valve.

It doesn't have any misfires, but does idle slightly rough sometimes.
It doesn't burn or go through any coolant, and so far as I can tell doesn't burn oil.
This weekend I will be doing a cylinder compression test and cylinder leakage test this weekend.
It is also possible that the valves may need to be re-adjusted since after I put the new heads on, I adjusted the valves while everything was fairly dry.

Keep in mind, that the fuel trims are only high when I take my foot off the gas. They are not high when I am rev'ing the engine as it would if I had a vacuum leak or fuel pressure regulator leak.

Any advice, tests or other help would be much appreciated.
Thanks.