Luke,
Back in the 60s - 70s - 80s car sound was not considered a big selling point to the vast majority of buyers, so sound systems & quality were not even close to what they are now. Having lived through all of those years and then some, I remember making changes to the sound on most of what I drove.
Back then, most radios had about 15 to 40 watts total amplification at 4 or 8 ohms. Domestic usually 8 ohm, foreign often 4 ohm. Replacing the stock speakers with some lower rated aftermarkets will give you much better sound but will not help much with total volume or sound intensity. In other words, the mirror ain't gonna vibrate. Also, even lower rated speakers will be under driven by the radio so after a certain point, the harder you drive them with the stock radio the more distortion you are gonna get. However, at moderate volumes, the sound should be far better than stock.
I know that they are not rated the best and are definitely not the most expensive on the market but i have been buying Sony decks for about 25 yrs now and have never been disappointed, except when I didn't spend that extra $40 to get the features I wanted. For any Sony deck over $140, the sound was great. (Audiophiles will argue, but most of us are not sound purists) I find that the Sony decks have as good or better sound specs as decks that are much more expensive.
As far as speakers go, for a lot of what is out there, you are often paying higher prices for the brand name, not the quality. For every day, decent, quality sound, I used to buy Blaupunk speakers. I don't know what they cost now but they used to be 20% cheaper than Kicker etc. but had the same specs.
Good Luck
