My 65 Shelby came stock with the straight bar - will work with the stock 14" diameter Hipo air cleaner or with a 2x4 Cobra kit manifold with Carters and small Stellings air cleaners. It will also clear stock Ford points-type distributors and will allow cap removal - but it is obviously less convenient than with the curved bar.
I'm putting the Cobra dress up kit option on my engine and this requires the curved MC bar due to the size of the air cleaner (like the 67-8 Shelby's as someone mentioned). The straight one is fine for the standard round air cleaner. I was told the MC bar and the Export bar are two of the best/cheapest initial things to do (especially for a convertible).
Thanks for the replys. It looks as though one of the POs was playing "Bullit" with my car and "Bullited into a ditch or perhaps a curb. As a result I have to replace a shock tower. I'm inclined to not trust the export brace still in the car and the monte carlo bar which will not fit anymore.