I need help with an overheating problem.  It all started on a small road trip about 35 miles each way.  it was a hot day about 90 degrees. on the way back i started to smell the antifreeze. so i pulled off and turned off the car and as soon as i did the antifreeze started to pour out of the radiator cap. At the time it did not have the overflow on. so it went everywhere. it seems that the back of the block gets the hottest visible smoke/steam)  I could only drive about 3-5 highway miles before it would do it again.  so i finally called a friend to come and tow us.  When it got back  to my house i checked the fluid refilled and started it aback up let it idle for 5 minutes then help the rpms at about 2500 for a couple minutes as soon as i let off of the throttle it started to dispense out of the radiator cap again. Since then ive been trying to figure it out, ive changed the thermostat, the waterpump seems to flow fine, the antifreeze level is ok.  the temp gauge does not work.  its a 400 small block stock besides headers, a th350 tranny, a 10 bolt with 4.10 gears, a stock radiator, and engine driven fan.  im not sure whats wrong besides that it drives fine.  i also add some aluminum flakes to stop a leak and maybe those clogged a passage or something.  some one please help, thank you  

First of all to get the radiator out of the way take it to get it checked. If that doesnt work it must be ur heads that are cracked.  

I would check the engine driven fan. if it's a clutch type, it may have went south on you and isn't doing it's job on cooling the engine. If it's not that, check for water on the oil dipstick, or on the bottom of the breather(if you have breathers) or on the bottom of the oil servicing cap on the valve cover it'll look like a milkshake. This will let you know in a hurry if you're getting water into the crank case, ie; cracked head or blown head gasket. If it's neither one of those, look at the cooling system. It could be the radiator is clogged or a water passage in the block. Get yourself a temp gauge that works, it'll save your engine in the long run. Overheating will do all kind of damage if not caught right away. Your mentioning the back of the block seems hotest(visible smoke and steam) would make me lean more towards a blown head gasket, that blew out to the outside of the block and is spraying water/antifreeze onto the headers(I've had this happen). If this is the case, it should be really easy to see that side of the block should be wet. Hope this helps? Good luck!!

I don't know if or not you had just put your engine together recently or not but, question 1: are the heads you are running for a 400 engine?   Remember 400 heads have a few more steam holes. Take a mirror and see if coolant is coming out of the back of the intake gaskets, sometimes the bolts will come loose

Check or replace the radiator.