Get a TD-mixvent duct fan made by S&P. I got the smallest one rated at 103CFM. Google it. I just solved my heat issue with it after three tries. My cabinet is slightly bigger than yours and I have about the same lights (150watt HPS and 4 23 watt flours). It's also pretty quiet. When you hook up those computer fans to ductwork with poor intake they don't move much air at all. Put your hand back there and see for yourself. I bet you are not getting anywhere close to 50CFM with both of them. The MIXVENT sucks air pretty good when used inline and I have shitty intake vents. Keeping my temps around 80 with all the lights going. I have found that ventilation is the hardest part of stealth cabinet growing. Big fans are too noisy and little ones don't move air. Stay away from those suncourt inline fans.

A cheaper solution might be to add another computer fan at the intake, But that's three fans making noise and it sounds like stealth is a priority for you. Me too!