[A1971Blazer]

The SM465 has always been a "truck" transmission...with the K/5 it's low 1st gear is ideal for off-road, slow maneuvering and also for holding back on the downhill trek.
If you don't go off-road then your best option would be to swap in an overdrive manual like the NV4500....and re-gear to a 3.73 ratio
some of the NV4500s still retain a low first gear but have the overdrive as well.
It sounds like you have a low gear already if it's turning fairly high RPM at 60-65 MPH

The only problem is that this would be an expensive swap.
You could also swap to an overdrive automatic....also an expensive swap unless you found a donor vehicle...cheap.
Both will require a fairly involved amount of labor....