One thing to take into consideration with ratios is that the tire size is a factor too. This is just a rough example, but you can have a 3.18 ratio with stock 13" tires and go to a 3.40 - and a taller tire and have the exact same effect (meaning no change). You can even, with a very tall tire, end up with an over all ratio closer to 3.00 than the stock 3.18 even though you have installed the 3.40 rearend. So, if you are not changing the tire size then everything Russ says is accurate. But, if you ever alter the tire size it becomes a factor.
Since you speak of future modification I'll throw out my example. I have a 2.3 Turbo, T-5 transmission and 175-70-13" tires. When I first swapped in my 8" it had 3.00 gears. 65 MPH on the freeway was 2,200 RPM. While all that sounds great at all the common street speeds (25, 35, 45 MPH) it was very difficult to find the right gear. By that I mean if I was revving too much an upshift tended to bog the car. It was never an ideal situation. I eventually got a 3.40 center section and the car is now far more driveable. It is happy in a certain gear for a certain speed. There is no more unpleasantness either a gear up - or down. Freeway 65 MPH is still a decent 2,600 RPM.
With the turbo engine I could probably go down to 3.25 and still be content. On the other hand if I go to taller tires (and eventually I intend to) the 3.40's are likely the better choice. Some like the 3.55 gears, but for an all around used I'd personally not go that far unless running very tall tires. For what is available "As/Is" from Ford I either recommend the 3.40 in the 8" on the numerically high end or if you are not building a high HP engine and can live with the 6-3/4" rear then 3.18 on the numerically low end. But, again tire size is a factor so consider it. There are all kinds of calculator available on the internet to see the engine RPM as it relates to the transmission final drive, rear ratio and tire size..