I don't believe the computer fans will create enough pressure to open the flaps fully...especially if you use dryer vent hose.
Have you actually measured yer under hood temps? I have with my thermocoupler setup...highly accurate/calibrated and fast changing.
Top number is fer the top of the engine/air plenum while the lower number is the temp nearest my oil catch jar.
I've measured about a 30°F increase above ambient with my grill blocked at 66% while driving. Not sure what it was without the grill block. I do know my under hood temps rise the highest after driving/parking and there's no air flow under there. I've seen it as high as 175°F.
To vent the excess heat...I just open my hood up fer about 20 minutes (at home only) and the temps drop about 60°F within a short period of time. It might keep some components from cooking too much...but it might be a moot point since there's about 350,000 miles on the truck already.
