Rebuilt 350 overheating any suggestions? please

There are so many factors that influence the running temperature of an engine that the more information you provide, the easier it will be to diagnose what is going on.

The radiator should be looked at. Brass copper radiators develop a white scale from the solder interacting with the coolant and that can clog the tubes. Once that happens, you really can't clean it effectively. If the radiator is old, think about replacing it. Aluminum radiators are the ultimate. One or two rows only. Stay away from any aluminum radiator that has more rows.

If the engine overheats on a steady highway run, that can point to a radiator that is clogged or possibly undersized especially if it cools down in stop and go traffic. It can also point to timing. In general, retarded timing will cause an engine to run hot, not advanced timing, unless the timing is advanced so far to cause detonation. You want to use vacuum advance so that your light load highway cruise timing is in the low 40's. Ignition timing seems to be a very confusing subject for a lot of car enthusiasts. Most know how to set the initial timing, and do so thinking they are done. There is a lot more to it than that. You should verify that the mechanical and vacuum advance systems work, and actually measure your total advance at WOT and light load cruise. Have a look at this thread,

http://www.v8buick.com/showthread.php?63475-Power-Timing-your-Buick-V8


Next important factor is air flow. It is really hard to beat the factory clutch fan and proper fan shroud. Not using a shroud allows air to go around the radiator. A free wheeling fan clutch will cause low speed overheating.

Anti freeze is great for freeze and corrosion protection, not so great for heat transfer. There is a reason that the back of the container warns not to use concentrations above 70%. Nothing cools better than 100% distilled water. You need corrosion protection. If you don't need freeze protection, you can use a product like Red line Water Wetter.

http://www.redlineoil.com/product.aspx?pid=74

Next, I have heard lots of instances of thermostats that are defective right out of the box. Unfortunately, you have to check them with a pot of water and thermometer. The stat should BEGIN to open at it's rated temperature. It should be FULLY OPEN 20* above that. That means a 160 stat should BEGIN to open at 160*. It should be fully open at 180*

Hope that helps.

 

It has to do with flat tappet cams,

http://www.v8buick.com/index.php?threads/new-oil-tech.222499/