STOCK ENGINE I assume, electric fan, is it installed as a pusher or puller, check to see if it is running the right way, does it have a shroud, Zirgo fans are way over rated and dont put out like they say. Fan should be installed as a puller mounted behind "engine side" the radiator and should come on about 175.
Check your timing also, on a stock 289-302 it should be around 8 deg with the vacuum advance pluged, also check that the advance is working but NOT at idle, if when you hook up the vacuum advance and the engine picks up RPM,s at idle then the advance line is hooked up at the wrong port connection.
Other things to check.
1) head gaskets on correct- stamped (FRONT) is on the front on both sides.
2) thermostat is in and working
3) water pump is correct and not a reverse rotation
4) pressure cap is correct pressure
I would be looking at the electric fan first, might be installed wrong or not big enough, then check timing.