Re-setting springs can be good, can be bad... depends on how well the job is done...
You can be sure that new springs will do the job for you and there probably isn't that much difference in cost. It is true that air shocks or helper springs or coil-overs will increase the spring rate. That means a harder ride in the back end. This can also lead to axle skipping or tramping on uneven surfaces.
You might also be able to find a new spring to suit standard ride height which has an additional leaf in it, this might improve durability (though the ravages of the next fifty+ years shouldn't worry you as much as those of the past 50+ years, should they?) without much penalty in spring rate.
Longer shackles will introduce new problems on to of the aesthetic one mentioned already. They will not be as rigid, so your rear end will be able to move around a bit that it can't now. And because of the increased leverage on the rubbers it will wear them more quickly.