Well I had the same problem with my 86 IROC with a 383. Put a big aluminum rad w/ a shroud and electric fan that would pull a ton of air through it.put a aluminum water pump on it with a restricter in the thermostat housing, and it always still ran hot. Same temps up around 250 degrees. We tried timing we tried checking the water flow we thought the battery may be getting weak and the alternator was checked to see if it was puting out enough charge to keep everything running up to speed. We looked at the plugs and they were perfect.We looked at everything that we could think of. Our soultion finally was a guy called and wanted to buy the car as a roller and we sold it to him that way. Another local guy wanted the motor for a drag car, we told him everythng and he has ran it for 2 years with no problems. Dont know what it was but that engine in that car = 250 degrees. :?