I am looking to use the car for straight line application,maybe a little mountain driving. I would have to say the exhaust manifolds are the "Log" style,very plain tubes.Actually my car came with a 390,but currently has a 343 under the hood now.It seems to handle the 670 very well?
For your 390 are you looking for mainly street use? Occasionally a weekend drag race up to 5500 rpm or so? Are you looking for reliability, improvement in horsepower (40-60 more) and mileage or is high performance your main goal? Sorry for the extra questions but it'll give me a better idea of what you looking for out of the 390.
Nothing wrong with the 343, I've had two and both ran strong. One stock, one with mild mods. I used the same carb on both, a 625 Carter (same on my 390s). For street use a 670 is plenty. It's real easy to over carb though.