Every thing should work by simply splicing in to the existing wires and will work with out the old ecm.
Your tuner should also be able to correct the pcm to make your tach function properly.
You can search this form or Google for "obs ls swap wiring" and it pull up tons of info for tapping into the trucks original wiring. Don't pay attention to the 96+ wiring threads as there different for us 95- guys. I do know for a fact the wires for the cel, tach, and speedo are behind the glove box (iirc the vss wire goes there as well.
There is absolutely no need for the old ecm, might even have problems getting your cel to function right if the cel wire was left connected to the older ecm.