i will have 2 or 3 amps in my truck to power my speakers. I want to remove them from under the rear seat and build some type of mounting device for them. i have an 8 cubic foot box for my SP4 - could i mount the amps on the side of this box or on some wood attached to this box OR would the vibrations be less than ideal for my amps? 

If you have room on the side of your enclosure for amps, then that would be ideal. Most people try to make the amp rack stackable so it wont take up much room. Really it all depends on the amount of amps being used, box placement, space available etc.

I have seen a many of amps mounted to enclosures without amp failure.

I've seen many an amp on the side of an enclosure.  It's impossible to know if the few that failed had anything to do with being mounted to the enclosure but knowing the people who had failures and how they wired their shit up it had nothing to do with being mounted to the enclosure.  

 

That being said I love a good looking actual amp rack in an install.  They can look so clean and gives it that much more of a custom look as anyone can just screw an amp to the side of a box.  However that's not always possible when space issues are a constraint as they may be in your case.  It took me awhile to figure out where to put and how to make the amp racks in both of my vehicles.  Lots of measuring, examining, test fitting, etc. and while the one in the van was luckily pretty straight forward the one in the Jimmy went through a few iterations before I finally came up with a design that would work.

 

All I'm getting at is sometimes you may have to think outside the box (pun intended) to get something that's truly unique.  If all that isn't your cup of tea, time unavailable or the space just isn't available then it makes sense to go to mounting to the enclosure.  Just my worthless .02 worth of opinion....

The box is the absolute worst place to mount an amplifier short of the engine block. If you must mount it there provide some isolation such as this:

http://www.soundsolutionsaudio.com/forum/topic/55861-a-well-though-out-15-mayhem-build/page-4

It all comes down to how much space you have. Well built amps are designed to handle vibrations. That's not to say that you can't damage an amp with vibration, but you can take simple steps to reduce or isolate the vibrations transmitted.

i have not designed a setup yet for the back of my truck. i have lots of room but am not sure exactly how i want to use it. the sub needs to be in the middle of the back seat so as to allow for proper clearance for excursion. outside of that i need to leave the floor of hte back seat open for tool boxes and work around that. its a project for this spring i think.