Easiest & cheapest way to gear up would be taller tires.
A Borg Warner overdrive from a '55-'62 GM car would be perfect for your 3.90 rear gearing, and is a virtual bolt-in. You could keep the column shift too. Only problem might be finding one in good shape. The later Saginaw overdrives are good too, but the shift linkage would have to change.
Best solution - but more work and $$ - is an overdrive T5 transmission from an S10 truck. Floor shift only, see "tech tips" for details.
