while the empeg is of standard DIN size, and will almost certainly fit in the viper, your install guy may have been referring to the dash opening trim. many manufacturers use a standard size DIN bay behind the dash, but make the opening slightly smaller for asthetics. many of us have had to carefully expand the trim opening with one of several methods (file, rasp, dremel, etc).

the end result usually comes out looking great, but you can't back out the install without replacing the dash trim. i'm pretty sure the FAQ has more info on this topic. good luck!

--dan.