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Some of the very early 240s had 3.91s and even 4.10s.
Yes, the guts can be swapped, but you need to seek the services of a qualified 4WD shop, or another place that has the same kind of expertise - it's not a shadetree job.
The easiest way to do it yourself is to swap the whole axle. Just undo the rear brake lines, the driveshaft, the trailing arm and torque rod bolts, and drop it out - put the new one in.
The subject of where to find which ratio is very cloudy, as often there were differences between similar cars even in the same model year. A 3.31 will only be found on a manual tranny car. Autos, both Turbo and NA, usually have 3.73s. The 3.91 was used on most of the old 3spd (no OD) autos, as well as most of the 242 GTs. The 3.54 is most commonly found in manual Turbos, and some V6s.
Switching gearing shouldn't take much off your range-per-tank, unless you do mostly highway driving. You won't necessarily be shifting more, but sooner - and you may find you like cruising around town in third instead of second. The biggest issue is the first gear ratio. Earlier cars had a slightly taller ratio that may work better overall, but I don't know of anyone who's actually done this kind of swap. You just have to decide if you can live with that really short first gear.
Paul Grimshaw's "Volvo Performance Handbook" recommends the 3.54 ratio/manual tranny as just about ideal for a NA 240, especially with a performance cam. I imagine if you do mostly in-town driving, and stick with the M-cam, a 3.73 would be quite liveable.
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