74 maverick 302 getting really frustrated

History:
rebuilt motor flat top pistons .030 over. really mild porting on heads (I took out the thermactor bumps)
Comp XE-262 Cam
Dual 2 1/4" exh.
E4AE mustang 4bbl intake and edel 600 carb.

Problem is over heating and boiling out the over flow!!!

I thought I had it sorted out last fall before the snow flew. I had changed out the stat adn a new rad cap on. I pressure tested the system it held to loss of only 1lb after about 40 minutes. then when I started it I ran it for almost an hour idling and the pressure came up to 12 pounds and temp stayed at 180. the took it for a seriously agressive drive and it stayed at 175 - 180. so I thought all was good.

I just put my plates on it for the summer and had it out most of the day and it was ok got a little warm once then settled right down. but then later today comin home from the local restaraunt and it pissed out most of the coolant out the over flow hose and temp shot up to 240 then cam down to about 195 still had heat then it woudl go up to about 240 then back down..

So am I the most unlucky person when it comes to Tstats or is my water pump weak or something? I have never seen anything like this.

I ran all the pressure tests and crap to see if it was head gaskets etc and it all seemed to check out. so now what? I am getting super frustrated with this.
I almost have a mind to ixnae the 302 and butcher the front and get a 390 into it or something.

Any thoughts?

 

If head gasket(s) were put on wrong then the steam holes are blocked

 

what kind of fan, is the pump belt good and not slipping, what temp stat are you running

 

X2!!! It's a common mistake, if the gaskets don't say "FRONT" on them....

 

It sounds to me like your thermostat failed. If the head gaskets were installed backwards you would never have run it for very long without it puking.
You could try running without the thermostat to see if it still overheats.
You can test the thermostat but for the cost just replace it.

 

I was having a huge problem with my car always heating up to 240-250. I changed the radiator and t-stat, still heated up. Read about springs having to be in lower radiator hose. Mine did not have one. Bought a flex hose, cut it open, put the spring in molded radiator hose and now stays at 180 all day. Check into that before putting big money down on other stuff, like I did.

 

Thats exactly what happened to me, the bottom hose was collapsing it would idle forever but when I drove it the temp would go through the roof.... I put a spring in it too...