I guess this has been answered many times before but be patient with me as I'm new to this site.
How much of a problem is it fitting 6"x9" speakers in the rear shelf of a '98? It appears there is a zinc coated plate that the current speakers are fitted to which if removed would leave an oval hole.....can 6"x9" speakers fit directly into this hole?

Should be no problems - have a look at

Some fit easily, others are a pig. I'd go with something that's known to work, such as what Tony's using.