i have an 85 z28 that came with a 305. i bought it for 300 dollars with a cracked block. i removed block bought a factory crate engine from atk motorsports. i put edelbrock intake, carb, a new water pump, hoses, gaskets. when i reinstalled motor i noticed it was running hot so i bought a bigger 4 core radiator for the car. my problem did not get solved so i bought all new equipment such as cap gauge thermostat water neck and i already had an aftermarket crank driven fan. the problem was still there. i heard about problems with these engines that others had so my dad was gracious enough to let me try his sooped up 350. well once again i still had problems. i adjusted the timing countless times timed it with the light and everything. so i bought a 16" reversible fan and mounted it on the outside of radiator to see if that would help the situation. so i pulled out the thermostat and still had problems. the shroud is fine and sitting good idont know what else to do if any one can help please do my email is noonespecial72883@yahoo.com please help

 

check out your sensors like your coolent temp,oxygen, or maybe your temp. gauge sending unit is screwed and you are running fine. The amount of coolant you put in also puts a big effect.
let me know how it goes

 

seems like you've gone through all the major causes, so here's some more to think about... don't take any offense to some of these, I just want to cover all the bases...

make sure you have the right temperature sending unit installed on the engine to compliment whatever guage you're using. if you're not using the right sending unit, that will cause erroneous readings.

intake manifold and gaskets - there are a few oddities among the several different types out there. make SURE the water jackets line up between the heads/manifold, and gaskets/hard parts. are they the right ones?

carb jetting - too lean a mixture will cause you to overheat.

oil grade - self explanatory.

oil draining from the heads? pull your valve covers and make sure you're not pooling.

water pump bolts - one or more of them extends into the water jacket, make sure it's not overextending and causing a blockage.

thermostat - is it in right side up?

try another timing light - set total timing, not just idle timing.

using proper fuel/spark plugs?

heater core bypassed? need replacement? does it still overheat with the heater running? does the temp change at all?

are you running the right mix of antifreeze/water? any additives?

is there any blockage at or near the radiator, a/c condenser (if equipped), tranny cooler, oil cooler?

if all else fails, don't be too proud to take it to someone else to look at.
surprisingly, my wife and kids have all pointed out things at one time or another that I just simply overlooked. you might try asking yours (if equipped). sometimes it takes someone who doesn't know everything to remind you that you don't know everything! good luck.