The 5.0 engines used in Fox Mustangs have proven to be tough as nails over the years, so it's extremely unlikely that the handful of extra horsepower you've added with the Cobra intake would cause enough additional stress for the engine to even notice, particularly if, as you say, the car has been well maintained and not really abused. The forged-piston 5.0 engines were somewhat notorious for oil consumption, at least in terms of what is considered normal for a late-model car (which is basically zero consumption). This is typical of forged-piston engines, as the fit between the pistons and the cylinder bores has to be a tad loose when cold to accommodate the extra thermal expansion of the slugs as things warm up inside the engine; hypereutectic pistons don't grow as much and can be fitted more tightly.
However, since you say your car didn't really use oil in the first place, the forged-piston consumption is probably not an issue in your case. It sounds like you're a victim of poor intake sealing. Although you state that the intake had no leaks, you probably mean external leaks-something you could see. What you can't see is the underside of the manifold, where the gaskets must seal the ports from the lifter valley. It's not uncommon for intake gaskets to leak at this point, even if they appear to have sealed nicely up top. When the leak occurs, oil is drawn from the lifter valley by the intake vacuum and sucked right into the intake ports. This also accounts for your slightly uneven idle.
If you pull the manifold, you'll probably see trails of oil on the floor of each intake port, or at least some of them, as one side may have sealed while the other did not. Most likely, though, if you didn't use any sealant around the intake ports (and many gasket instructions advise not to), all the ports are probably drawing oil. The smoke from this could easily be slight enough that you might not notice it unless you could watch the tailpipe at all times.
The solution, if this proves to be the case, is to simply reseal the intake with new gaskets. You may want to use thicker, composite gaskets, like a stock replacement type, rather than the thin, hard performance pieces. There may, for some reason, be a slight mismatch in intake surface angles, and the softer, thicker gaskets will help compensate. A thin film of sealer-make it petroleum friendly-may be advisable as well. When you fix it, your idle should also smooth out.
Vibration IrritationI've been chasing an annoying shake/shimmy in my '69 Nova 350 with four-speed Saginaw (used to be a 250/Powerglide). The frontend has been aligned and the tires balanced, and I've installed new inner and outer tie rods, ball joints, and so on. The only things not new are the centerlink and steering box. At first I thought this 55-mph shimmy was related to the front of the car. But after doing the alignment, it was still there, and then I started to notice that it did it in every gear at around 2,500 rpm. I only really notice it in Fourth at highway speeds, because I'm obviously in that rpm range for a much longer period of time. Is it my worn body bushings, or a worn transmission bearing (never rebuilt)? I can't sleep when something is wrong with my car.Scott FrymoyerNorristown, PA