The spacing of the holes is staggered on stock Mustangs. Some vendors sell "even spaced" ones for Shelby applications. Not sure if its a 65-66 thing or all...
Note below, the spacing between the two middle bolts is diferent to that between the outer and inners on each side. Odd spacing.
On the "spring" an export brace is also, technically, a spring, as it flexes just as a curved MC bar will. IMO the flex in a curved MC bar would be minimal, and it will keep the towers from flexing somewhat, better than not being there for sure.
As for the benfit of an export brace, the export brace is solid at the firewall it resists flex better than the stock bar, as it resists moment in the horizontal plane. The stock bars bolted to firewall and tower do not have this restraint (well, a bit, but nowhere near as much.
Imaging holding the end of the stock bar at the firewall and having someone else try to flex the tower end in or out. Would be very hard to hold as your end has no lever arm (ok, very little, the span between the two bolt holes) to resist moment. Do the same, but with you on one tower end of a Export brace and the other person at the other tower. Now bolt the back edge to the firewall to stifen it even more... Very different dynamic, as you now have a very large and quite stiff lever arm to resist the motion...