take all the variables out of the equation.
build the machine outside the case with 1 single fan on cpu cooler and verify the air moving in the direction you think it should.
The contact between cpu and heatsink is by far the most important part of the setup as far as temps are concerned.
Buff the cpu and sink contact areas with specialised cleaner or IPA (no, not the beer) to a high shine.
Spread a thin film of thermal compound across the surface of the cpu with a credit card or similar so it is uniform and thin. Take your time with this, it works much better than a dollop or a line and then relying on it to spread evenly when pressure is applied.
When fitting the heatsink you should feel it making contact with the compound. Fix it on loosely and give the sink a slight twist, just a degree or two in either direction, you should feel it sliding ever so slightly.
Tighten her up and you're ready to go. 75 should be what you're aiming for.