The torque multiplication isn't a variable - the engine must be turning very close to its power peak, and you calculate the gear ratio backwards from that. What exact ratio is, not really important.
An OD, in this case, would be no use since it slows the engine down even more. The exception might be if there were an even higher numerical set available (4.50, etc.), and the OD would allow an extra gear - but only a real advantage if the 4-OD shift is closer than your existing 3-4 (which is terrible).
When you shift the engine falls off to 5,158 RPM. It's important that any development work you do doesn't create a major hole in the torque curve above this speed, even if it reduces peak power slightly. This shift will occur (4.22:1 ratio) at 91 mph, and if you don't have enough power at 5,158 to pull up above 91 the car won't go any faster. I don't think it's likely but moving the torque curve around with resonance etc. sometimes creates some strange effects - don't trip over one. You don't need a dyno to detect this - just accel in 2 or 3 from 5,000 and see if it picks up a lot suddenly.
There are close-ratio internal sets for this box that move 3 much closer to 4, which is what you want. Typically, they compromise 1st quite a bit but that's not much harm on a long course. If you haven't chosen a cam or tuned intake/exhaust lengths yet I suggest you explore your power spread vs. gear ratios before making a final choice - closer ratios means the power peak can be narrower.
