Overheating Problems

[Madlx5.0]

I converted a 4cyl mustang into 5.0 with rebuilt 302, about 2 years ago. Every since I got it running, it has been having overheating problems, mainly in spring and summer months. Happens while driving on highway and in slow moving traffic. I have replaced water pump, t-stat with 180, radiator cap with spring release, bought new 2 core radiator from o reilly back in 2005, flushed radiator out, replace hoses, installed proform 16 in electric fan, and burped cooling system. Timing is set at 10 degrees advanced before top dead center. No matter what I do, when the weather is above 70 degrees, temp gauge will go to 180, 210, 230, 250 while driving. Stays at 180-200 idleing in driveway. One thing I did notice after flushing, have brown liquid in radiator. I'm thinking my water jackets and engine block may have rust and corrosion. IS there anyway to clean block and water jackets, like at radiator shop or something performance shop can do .
Any advice or recommendations!!!!!!!!!!!

 

[65ShelbyClone]

Are you using the stock temp gage? If so, that's a big part of the problem; you don't know how hot it's actually getting.

 

[93GTosu]

is there an air deflector under the radiator? its a black piece of material that hangs down and helps to "deflect" air up into the radiator. Sometimes if you don't have this you can get trouble cooling while moving (which is what it seems like by your post)

what kind of "brown" was in the radiator? brown like oil? how's your oil look? head gasket failure?

just throwing ideas out there.

 

[86bluecobra]

Could you have the wrong rotation water pump on the car?