[Hcb3200]

Agree with MR 48 here.
What temp is it running on the road at speed and at idle?

My 6.0 LS in the 1956 runs at 188 - 195 and I can't get it higher unless 100 degrees outside I am in traffic and idling without the fan on. Because its not packed in with a zillion other things in the engine bay like modern donors have.

Stock in my 3 other LS vehicles of modern era is running around 200 - 210 all the time. 2006 chevy avalanche, 2012 Yukon, 2015 Silverado

Note. the 56 with the LS actually runs better when it gets up to 200 and I am in traffic. Because why. The computer wants it there. unless you have made those adjustments in the parameter in the programing then it will like to be in the range the original donor wanted it in. To cold is just as bad as too hot with the computers. They want all the numbers to line up to what all the settings are for optimal performance (or fuel economy) depending on the programming.