So my question is how big are you guys drilling holes in your thermostat? Are you placing it is it's most vertical position in housing? Idea is to provide an air bleed to remove
air from high point in system, and
not as a coolant bypass hole. 1/32nd (.031) drill is certainly big enough. I typically drill them even smaller. Either that or actually
use the 3/8" vent plug on 8V heads. Doing that, you don't need a drilled thermostat.
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Those running the TU's radiators in L-bodies, the coolant return hose aluminum fitting included with the l-body kit I noticed is very restrictive. Probably less than 1/4 the size of the factory GLHS 5/8" barb on tank. Didn't use this fitting because of this.
Can always place something in front of radiator like the big rigs do in the cold, if your suspecting the heat exchangers are more effective than the engines ability to produce and retain heat. I do that on my New Yorker during the winter months. Works great.
Only time I've ever experienced wild temp swings on gauges is when heater core is partially blocked, or someone isn't running any bypass at all. Then scenario like Wayne and JT talked about in post 4 & 5 comes into play.
Todd