Ignition Timing Adjustment/Reset?

[Coel]

I have my 06 LS430 that I just did a complete engine overhaul, I just started it today and it struggles to start but can get started. It idles very rough on idle/high rpms and dies if gets below 1000 rpms, I removed the cam covers to check if I got the timing wrong but everything is on point. The engine light came on and stayed flashing, I turned it off immediately and scanned the codes it had and P0300 through P0308 so all cylinders are misfiring. I then took a timing gun and attached it to #1 coil pack with a sparkplug wire and flashed the harmonic balancer and saw that the flash was WAY off by atleast 20-25 degrees. My question is how can I move the ignition timing to 0? I know these cars cannot be adjusted like a carburetor and my car is OEM all the way, no aftermarket ECM. So what could cause this issue?

Other jobs I've done to this car are. I shipped out my ECM a year ago to get it rebuilt because it was causing issues to my transmission (rough shifting) and it fixed that problem. And the reason for the over haul, after a compression test I found that the valve seats broke on #7 cylinder intake, so I pulled the engine rebuilt it and put it back in and Im here now.