OEM speakers were 10 ohm. As mentioned a true 8 ohm speaker will work OK with the original radio, but you have to be careful since many aftermarket speakers these days are marked '4 to 8 ohm' but are actually closer to 4 ohms than 8.
Removing the stock radio isn't as hard as it may seem, once you remove the glove box you have pretty decent access through the glove box door. If I recall correctly there's just a single mounting bracket on the back side of the radio, and then you take off the knobs and loosen the retaining nuts on the volume and tuning shafts.
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