That first gear/rear end "product" ratio would be for best 1/4 mile times. Are you going racing?
Unfortunately, all the serious Cobra racers that I knew of are no longer around to answer that question specifically BUT when you are serious about it, there are also other factors that enter into the equation such as disconnecting the front anti-sway bar and using uplock front shocks.
How you do that in a 427 Cobra and not get killed with sudden snap steer is way beyond me. I know that just putting slicks on stock Cobras in the day opened a can of worms and I was just a kid that was the fly on the wall that everyone was attempting to swat with the fly swatter!
I was just "the Joe College kid" with white leather "Joe Namith" Puma sneakers, looked like an under cover Cop that no one would talk to, just mutter under their breath about. That's how the east coast racers were back then. No one talked less they get 'busted' for something?
I can add a tangential observation on a car that had similar f/r weight distribution but not the same amount of off the line torque that perhaps one of the ultimate Pantera builder guys here can check in on?
Back in the day, Gary Hall took one of his Panteras, put 5.22 gears in the ZF and told Mike Cook to go out and try to blow it up. The car immediately dropped down into the 9's. I don't remember the trap speeds off hand and Hall didn't talk about other chassis changes they had to make but I did talk to Mike about it several times.
I believe it was a non-stock block and I know it had A3 heads on it. Other then that, and the 5.22's, I have no other details. The sound of the engine was definitely a 8,000 rpm rev sound but there was no tell-tale tach mounted to spill the beans.
I at one time ran that question of first gear/rear gear product ratio past Randy and I got no response to it. I let it go since I presumed that since he IS, not was a C4 guy, he might look at it differently but as he departed here, he was working on a different engine/drive line combination looking to drop his '66 GT350 into the nines. I haven't heard anymore on the progress since.
MY experience with a top loader, a wide v. close ratio is that on a street car, you want the close ratio. The reason being that the rpm drop between first and second is too drastic to make the car "enjoyable".
I think that back in the day, "Liberty" was combining the 2.78 into a close ratio box 4 speed intended to match up with 4.88's.
IF you are intending to apply all of this to a CSX3000 something original car then the close ratio would be something to use. However, IF by chance you were playing with alchemy my suggestion would be to consider a more practical transmission selection to go with the 3.31 rear of a 5 speed Richmond/Nash. That is typically a 3.26 first gear with a T10 close ratio spread.
I long ago went that route in my GT350 with 3.50 rears and after 30 some years is still running. Everything including the speedometer drive just bolts up.
The only thing you would need to mess with on it would be a shifter mount which is one of the simpler things to do, and maybe the rear transmission mount which varies somewhat from the Ford top loader.
The tail shaft configuration on it more closely RESEMBLES that of the Ford tail shaft so there is SOME flexibility there in mounting the shifter v other 5 and 6 speed transmissions.
I would suspect that with a BB Ford engine, a six speed with it's overdrive top gear could be a breakage issue but can't speak to that combination scientifically?
That transmission, the Richmond 5 speed, uses a Chevy fine spline 22 input, a big Ford 31 spline output and yoke, is a Ford bolt pattern and is the 24" Ford length so it adapts itself well.
The only thing I had issues with were the friction welded shifter inputs that I promptly snaped off first round. That was a simple fix with billet shifter cams which should have been in the box initially to begin with but were part of the "fun" of the learning curve.
So that's what I know and perhaps there is something in it that you can find useful for your application?