All my Cobra friends say 4:10s or higher. 3:55s would be a waste even on a GT. I think he needs to consider how he plans to primarily use the car...drag racing or auto-x/road racing. Lower gears (4:10) for auto-x, higher for drag racing.
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"Back in the day"-Red '66 289
"Later"- Black '68 289
"Today"- White 2000 GT
Stock except K&N & Flowmasters
ET-13.952@98.87 RT-.507 60'-2.09
Son #1 - '87 GT stock
Son #2 - '89 LX 5.0 stock