Just turn it down. At full class A bias the circuit is capable of 8.9 watts into 8 ohms. Not all transistors could take that so many were set lower. Realistically it is hard to hear the difference between 8.9 watts and 5 watts. Plus, there is some amount of DC on the speakers so if they are not stock they might not like that DC. I know I blew a pair like that a long time ago.

If you check the DC resistance of the output choke you can do a few calculations. It is probably 1.2 ohms and they call for one volt across it. For the stock speakers that were actually ten ohms, the DC did not bother them, but if they have been replaced you want that bias lower or else put a blocking capacitor in series.

I would say that you don't want any more than a volt across that output choke. If you want more power get a power booster. Also, since it failed ad they never do, and the age, I would suspect the cap across that output choke. Those are alot higher value than we are used to these days, I don't remember specifically but they might be 10 uF or something like that. If that goes open the output transistor is very vulnerable to inductive spikes. Being across the choke, very few caps checkers will check their ESP, bitching about a short circuit. So you have to disconnect it. My scope based one will but those mean some building. You have to have a known impedance of a known voltage of a square wave and that requires prints. It might be useful later but not enough for just one project.

Anyway, using the output choke DC resistance as a guide, set it to somewhat less than one amp and see how it works. It might be just fine. I the speakers have been replaced they might be four ohms or at least have a lower DC resistance in which case you have to set it even lower. If stock speakers which will be ten ohm, put a bit less than a volt on it and your output transistor will be dissipating about 12 watts all the time. Even the cheaper ones can handle that.

There were some of those cars that had faders and more than one speaker. Those had the bigger output transistors and the heftier chokes, and were of course biased higher to get that couple of extra watts.

Check that cap, and then get say 750 mV across that coil and see how it sounds. If it sounds good, ship it.