You can determine the new speed/rev relationship by multiplying revs by the ratio of the new to original axle ratio.
For example, if you are turning 4000 revs at 80 with the 3.46, the revs with the 3.07 will be:
4000 (3.07/3.46) = 3549,
about a ten percent reduction.
The SLC is a fairly heavy car, so with only a 3.5L engine it needs short gearing. U.S. spec 450s can pull a taller gearing because they have more torque.
If you were to make the change I would not go "taller" than the 3.07. (The 2.47 is much taller.)
The 3.07 will reduce acceleration, but it will probably be liveable. With the 2.47 you will notice a considerable loss of acceleration.
The other issue you want to investigate is the speedometer. Changing the axle ratio will upset speedometer calibration by the same ratio as stated above, so you need to understand what changes must be made to bring it to proper calibration with the different axle ratio.
The 3.07 gear will probably not provide more than 5 percent better fuel consumption, and wear is not that much of an issue. Higher revs means lower torque required to maintain speed, so the difference in engine longevity between two gear ratios ten percent apart is not significant.
The biggest reason to change gear ratio IMO is to reduce engine noise at freeway cruising speed, but with a V8 the difference will not be that much.
Bottom line, don't set your expectations too high for what you'll gain, and for what it will cost, it might not really be worth it.
One other observation. Assuming your top gear is 1:1 with a 3.46 ratio your tire revs per mile would have to be 867 revs per mile to be truning 1000 revs per 20 MPH. If I recall correctly your OEM tire size is 205/70R-14, which are about 813 revs per mile, This leads me to the conclusion that you have either a shorter (high numerical) axle than you think or your speedometer is reading close to ten percent low.