My freshly rebuilt engine has been overheating since I put it in. I just replaced the 6 blade 17 inch static fan with a 19.5 inch 7 blade clutch fan with a new fan clutch, but the car still heats up very quickly. When I took the car out for a test drive, the engine rose to 240 degrees in about 15 mins while driving around the city. It rose faster while idling. When I shut the car off the temp quickly raises to 250 or above (my tenp gauge only goes to 250). It acted similar with the 17 inch fan.

After shutting the car off, I could hear the fluids making a gurgling sound in the engine.

Here is some more background:
1) This is a 68 firebird that came originally with a 350 auto trans and A/C. The A/C was gone before I got it and now it has 73 455 that is newly rebuilt. I changed the trans from an auto to a 4 speed.
2) The water pump is new
3) The radiator is the original 3 core automatic trans unit that worked fine with the previous 350 engine. I changed the trans to a 4 speed and plugged the trans line holes in the radiator.
4) The timing cover is from the 68 350.
5) The backing plate is somewhat pitted, but has no holes
6) The top baffles are in
7) I have a shroud.
8) I changed the fan from a 6 blade 17 inch to a 7 blade 19.5 inch. The fan fits about 1/4 way into the shroud.
9) I added water wetter and have a 50/50 mix of radiator fluid and water.
10) The heater core is new.
11) The water line is 3 inches from the top
12) The radiator did not spill out when the temp rose too high or when I shut the car off.

Now that I have the proper fan in, any thoughts on where I should take this troubleshooting next?