Update on this after going out today.
Moved the bleed tubes to the front of the LT1 last month to hopefully solve overheating spikes. I just used the bleed fittings and then connected across the engine with 1/4 inch hose.
Took the boat out for a full warmup then read the temps with a direct reading thermometer.
The boat held steady at 160 on the dash indicator and about 164 at idle.
Cylinder head temp drivers side after extended idle
And passenger side head temp was the exact same
Passenger exhaust riser after extended idle ran at 115 F.
Drivers side riser ran hotter at idle (165 f) and pretty much matches cylinder head temperature
Once underway the riser temps would even out with the drivers side cooling down to about the same temp as the passenger side. Once at idle the drivers side riser would slowly rise to match the cylinder head temp. Both lower manifolds stayed even temp wise, it was just the drivers side riser. The bypass from each head on the LT1 dumps into the driver side riser and at idle the hot bypass water brings the temp up in that riser.(That's the bypass hose in the last picture) Once underway the greatly increased flow through the manifolds evens the temps out.
No more temperature problems or spikes were noted, nice and stable temps coming off plane now. Keep those bleed tubes clean if you own an LT-1.