The New Engine runs at higher temps 3/4 Gauge.

[67428GT500]

I put 20 miles on the Shelby's new engine last night. The other engine had C6AE-R with about 11:1 compression.The Edelbrock  heads and JE inverted dome pistons have me at about 9.9:1 compression.  I have the Edelbrock water pump as I did before and a ford factory radiator with a heavy duty three core. I am running the factory fan, shroud and and clutch.
I have changed to Edelbrocks, but everything else pretty much remains the same.  The other engine seemed to be a constant at about 1/2 gauge or just below.  It seems to come up to temperature much faster as well  I have the high flow Mr. Gasket Thermostat that looks like the Robert Shaw. 
It is also supposed to be a high flow. I did accidently drop it during assembly. It didn't appear to have damage. Are is the Mr. Gasket a decent unit, or are they Chinesium garbage that tends to fail?

[My427stang]

Could be the  thermostat, could just be a fresh and tighter engine building some heat.  Is it the same sending unit?  I have seen them vary widely

Also, are you sure it's fully purged of air?  I generally drill a hole in a thermostat now, but if you don't it can be a little bit of a pain to fill and do cam break in

[Heo]

I take of the heater hose on the intake and fill to water come out
then you know there is no or a minimum of air left in there

[Drew Pojedinec]

Get a temporary mechanical gauge.  I know it's a Shelby, but just zip tie it in place for the moment, you may be concerned about an engine running normal temps.

[frnkeore]

I would put both the old and new thermostats in a pan of water, heat it and see when they open. Or just put the new one in and measure the temp that it opens.

[machoneman]

Hole in t-stat rim, jack up the car until the fronts just touch, fill via the heater hose. All 3 will purge air from the cooling system.

On edit: heater on full blast as well.

[BruceS]

A few months ago I did what Drew suggested, and put a temporary aftermarket water temp gauge under the dash on the Galaxie after installing the 482.  I originally had been using the Ford temp gauge which on a '63 Galaxie is only calibrated C to H.  The temporary gauge revealed I was running normal temps (180-195). So I've gone back to the Ford gauge and quit worrying!   

[67428GT500]

The thermostat I bought is a Mr. Gasket, which is a Robert Shaw knock-off.  I remembered dropping it while installing it.  I heated a pan of water to 180 and as high as 220. It doesn't open completely. It's dragging.  I am going to pick up the EMP Thermostat at Summit. 
I also found an NOS Ford sending unit which I ordered from E-pay.  It seems the aftermarket temperature senders tend to be all over the board based on feedback from several Forums.
What I do know is that I have 45 miles on the car with no loss of coolant apparent in the radiator tank, but the radiator top tank starts to feel hot almost as quickly as I fire the engine.
I can only assume that the thermostat isn't regulating coolant flow as it should. The upper tank used to stay cool to the touch until it came up to operating temperature.

[My427stang]

To be honest, I just buy local Stants and drill a hole at 12:00, then I glue it in place to make sure it stays at 12:00 right at the edge of the casting

I haven't seen any benefit of any other kind.  As far as the senders, it may still be off, no two seem to read exactly the same