It's a little more complicated than that. Most of the early (pre-'80) cars had taller ratios like 3.91 and 3.73 regardless of tranny type - usually a 3.91 for a 4-cyl and a 3.73 for the six-cyl. Most '80-on 240 non-turbos had a 3.31 if manual, 3.54 or 3.73 if auto. Turbos usually had 2 3.54 if manual, 3.73 if auto. But there are more permutations than I could possibly remember - the only way to be really sure is to check the label on the axle tube, or better still, turn the wheel and count.

Yes, any Dana 30 rear will fit. The easiest swap is to just take the whole rear axle assembly from another 240 - just bolt it up to the trailing arms and torque rods and connect the brake lines.

A 3.54 rear is a great all-round ratio for most setups, especially if you've done some mods to increase midrange power - it might not be worth all the trouble of the swap for such a small change, though. A 3.73 will give you noticable results w/out being ridiculously short. A 3.91 would be a great off-the-line performer but fuel economy would definitely suffer, and you'd be shifting quite often in city traffic as you squirt from light to light.

I have a 3.31/M46 combo in my 240. When the time comes to swap my rear, my choice will likely be the 3.73 - though I may consider a 3.91.