crazy8s you are going to run into all sorts of issues with that mach 460 system. Here are a few things you need to know.
Mach 460 systems consist of a two way "component" type speaker system, there is four 6"x8" speakers and four separate tweeters.
But UNLIKE most stereo systems the Mach 460 head unit(deck) only powers the four tweeters and it only produces high frequencies. There are two amps in the trunk, one amp powers the 6X8 door speakers and the other amp powers the 6X8 rear speakers, the amps only produce mid-low frequencies. All the factory speakers are 6 ohms.
So the 6x8 speakers in the door and rear only produce lows-mids range sound. They are just standard speakers, no tweeters attached like aftermarket 6x8s.
The only correct way to replace the speaker is with a OEM factory 6 ohm speaker. Most people just replace everything with aftermarket products. I replaced all my speakers with Alpine SPX-17REF 6.5" component speaker systems. They included 6x8" to 6.5" speaker adapters so you can mount 6.5"s in the factory location and I mounted the Alpine tweetes in the factory tweeter location with a adapter I made from lexan I bought at Lowes.
You can try and replace the 6x8s with another set of aftermarket 6x8s but you will have a few issues which will result in poor sound. The main issue is the fact that the factory 6x8s are 6 ohms, if you replace them with 4 ohm aftermarket speakers the 4 ohm speakers will be louder than the other factory speakers. The harmonal balance between your 6x8s and your tweeters will be destroyed. Connecting 4 ohm speakers to the factory mach 460 amps will also lead to premature amp failure.
If you are not ready to switch to aftermarket equipment it may be best for you to fing a OEM factory replacement speaker. You can almost always find them on ebay, new and used. If all you need is a replacement and you can't find a good deal on ebay feel free to PM me.
Big Thanks to ALLFORDMUSTANGS!