car temperature running hot

260-270????? That is REALLY hot. Ideal temp for a mustang is around or below 200 degree's.

Are you taking this reading from the stock gauge? If so, i'd get an autometer gauge hooked up asap so you know the real temp of your motor. Stock gauges are inaccurate as hell, they're garbage.
 
Sorry to hear about your problem. I live in AZ so i can relate. No, that high of a temp is not normal. Did you cheak the coolant recovery tank. If it is empty cheak your radiator. that shop may have forgotten to put in coolant. If it is full then the thermostat may be backwards. Assuming this overheating wasn't as bad as it was before you took it in. (of course is was to a degree. You did have your water pump replaced). If the recovery tank is full, is it boiling? and smell of exhaust gases ? does your Engine oil look like a milkshake ? If so, then you have a blown head gasket. Or it may be simply the temp sending wire is resting on the block it self, not connected. I don't see why the shop would have toutched it to begin with. Cheak these things out a let us know.
 
That's waaay too high...especially with a new pump and thermostat. Not sure if this applies but perhaps the thermostat was placed in the housing backwards. This would cause your temps to reach that high. I'd definetly bring it back and have them check it out.
 
What kind of cooling fan do you have, factory or electric? If factory you should try to spin the fan blade with the engine off, there should be some resistance, if it spins freely the clutch is gone. Take a look for any oil leaks on the inside hub of the fan to see if the clutch is leaking oil.
 
after checking everything that all of you suggested, i found that the radiator just was so badly clogged that it wasn't getting enough flow to cool the engine properly. after installing a new radiator it runs like a dream again, thanks to everyone who replied.

Ben
 
Ben, im glad to hear you got it whipped. i would still advise you to get an aftermarket gauge. i kinda doubt that you were truly running that hot. since you have had issues and live in a hot climate, i would want to know what is REALLY goin on, temp wise.
again, glad to hear you got it whipped.