Want to convert from T5 to auto transmission.

[750hp]

Vacuum - no issue because it's only relevant at wide open throttle. I'd still recommend you talk to a gearbox shop about getting a manual valve body and shifting manually.
Smooth - C4s need to have reasonably high line pressure to prevent burning out clutches when shifting from 2nd to 3rd, however.....
Stall Convertor - as Joyridin said, you'll need a higher rpm stall convertor than stock, so on the street you can shift under the stall rpm speed at partial throttle and it'll shift like a stock vehicle.
Stall speed/gears - my thoughts are a bit different from Joyridin regarding gearing and stall speed. Cobras are light cars so you can get away with a tighter convertor. With 600ish hp I would think about a 2800-3000rpm convertor would be more than enough and you wouldn't need lower ratio than a 3.5:1 rear gear (ie: you don't need a higher number than this).

65mph might be at about the convertor lockup speed (some assumptions about tires and gearing) but even if you're going down the highway at 2400rpm with a good quality 3000rpm convertor, it won't be a problem. You wouldn't even notice that it was driving "under the convertor" unless you gave it a quick hit of the throttle. Then you'd see the rpm flare up to 3000, but at part throttle the rpm won't be flaring up.

Shifter - can fit it where you want it because it's remote mounted and cable operated.

If highway rpm are an issue, you could fit a Gear Vendors overdrive, but it's not cheap and I don't think it would be necessary.