I also have to go with the type X components (which i just recieved this past week). The crossovers are quite big and would be a shame to hide them behind some panel. With the type x components, you can adjust the actual crossover to accommodate for speaker locations and on/off axis compensation by simply placing the jumpers to the desired settings which are illustrated in the instructions manual. For imaging reasons, I prefer to have speakers in the rear but keeping the gain low so it does not over power the fronts. There are settings on the type X crossovers for rear deck applications, but you would have to modify the speaker locations to fit either the 5 1/4" or 6 1/2" speakers.
Personally I believe that the SPX-F--- speakers are Alpine's top of the line speakers (conventionally speaking). With a retail price of about $3500.00 the SPX-Z18T speakers are a little far fetched for a speaker system that HAS to have customized enclosures.
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another picture of the SPX-F--- Crossover