Cooling problem

I know cooling problems are brought up quite often and get kinda old. I think my problem is a little differnent in the sense that I have found no answer and it involves a converted s10.

I have an 87 s10 ext cab truck that I converted to a v8 about three years ago. Cooling has been very hard to do since then. For the most part it was no big deal because the truck was in the woods all day at idle and keeping the truck cool at idle is not a problem. Highway speeds however are different.

Basicly If I put on a manual fan and run good highway speedsit runs cool and run hot at low speeds and idle. Use electric fan and I run cool at low speeds and idle with highway speeds running hot.

I replaced the enginge 2 months ago with a mild 350 and at the same time bought all new cooling peices. New summit rad, matching summit shroud and electic fan (Summit tech support said that combo was more than needed to keep small block cool) New mildon high flow water pump, New high performance 160d stat and solid metal rad hoses. Evey cooling peice is brand new. At low speeds and at idle It maintains 180 degrees (even though the stat is 160) At highway speeds the faster I go the hotter it gets. I thought about it and tried a few things. I made cuts in the alum fan shroud and made flaps so they will open at highway speeds and allow more air to be forced through the rad. Did not help. I took the truck out the other day and it was about 50 degrees out and at 80 mph it kept at 180. Now what does that mean? I have tried to think about everything from slowing down the water pump to speeding it up. I use v-belts and the rad is a 2 core with each core being an 1 1/4. I have no room to run a manual fan plus electric. Most of the front end has been cut out to fit the 27 1/5 x 19 rad. The tranny cooler and oil cooler are both very near pushing the front grill out. So the is not much as far as room to try to run both electric and manual fans either way. Does anyone have some insight? I am really wondering now if the outside temp can tell me something. If it is cool out and the truck runs cool does that mean that when its 70 out and it runs hot at highspeeds that the water is flowing to fast to be cooled? Whats your take?

I would like to add that the temps I get are good. I have both manual and digital temp gauges. I also have a temp gun that I have used to check the temp on the rad and and such. Which is funny cause the rad is always very cool and the high temp is always at the termastat houseing.

mike