Mohican question, capacitors.

[jebem]

If I may, and for the sake of documenting as many aspects as possible about this issue:

Why did the radio designers used high impedance speaker drivers (like 35 Ohm here) in the 60's radios?

It's about saving money.
They traded the audio output transformer for a high impedance speaker, and at the same time they kept using small current Germanium transistors.

The used 2N407 with its maximum current of 70mA is not really a power transistor able to drive direct speaker loads as we see as the standard design since the mid 70's.

Remember, in the 60's it was a standard practice to use two audio transformers, one driver (as used in this radio), and one output (missing in this radio design).

So the obvious solution here was to use a high impedance speaker driver, in order to maintain the output transistor's current as low as possible and under its admissible operation maximum ratings.

If they had employed a output power transformer, it would be able to use low impedance speaker drivers, without overloading their choice of output transistors.

They also could have used a low impedance speaker driver if they had selected Germanium power transistors.
However that solution would be more expensive than using an output transformer (Remember, in the 60's the Germanium transistors were replacing the valve/tube technology but they were expensive specially the high current power versions).

So, on this specific radio design:
The issue I see on lowering the speaker load is that the output circuit will be current overloaded and this usually will result in an large increase in distortion.
That output circuit was not designed to work with low impedance speakers.