I agree with Westkoast EA, custom tailshaft, u may be able to get away with a ED XR8 Sprint, tailshaft, or the EB gt one, (i think there mechanically similiar, they share the same motor) if that will fit, (i do doubt it, but doesnt hurt looking into it). Have u tried using a rear spoiler it may actually stablise the car abit more at that speed. But once again im commenting on something i have no idea about so dont quote me. Yeah these mods will only be good for the 200kph limit, anything over your going to have to look at a custom tailshaft.
P.S. I dont think an EA would hold together at 300kph (my guess is it will vibrate apart not far from 200), any car that hasn't been balanced for racing, in the production stages is generally not going to handle too well at that speed, if u even get it there.