I was in a similar situation when I wanted to mount my empeg in my 2000 Silverado. I wanted to use the stock head unit and speakers, since I was pretty satisfied with them.
There is quite a dilemma though. To use the internal amp in the factory head unit, you have to put the empeg before the stock hu, but the stock hu does not have aux inputs. There is an adapter from peripheral that goes into the 9-pin plug on the back of the stock hu (where your tape goes now) which gives you the option of aux inputs. The problem though, is that I think you have to have a cd changer hooked up to the adapter in order for it to properly emulate the factory optional cd changer. Since you have an empeg, you have no use for a cd changer, so that is not really a useful option. You could put the empeg after the stock hu, as the empeg has aux inputs, but that would require using an external amp. That also seems like a bit of a waste, since the stock hu has a pretty good amp built-in, but it may be the best option.
One last idea I can think of, is to cannibalize the tape deck. I dont know the pinout of that 9 pin harness, but perhaps there is simple analogue audio on that harness. If that is the case, then you could splice the empeg outputs into that harness, and leave the rest of the harness connected to the tape deck. You would have to leave the tape deck connected though, so that the factory hu 'sees' that there is an active input and accepts audio. I would think you could find somewhere behind the dash to hide the tape deck.
These are probably not the answers you are looking for though, so hopefully someone else here has some better ideas. Good luck.