GD Dash swap in an RSTI

Now, admittedly I haven't scoured the site for this information so please don't rip into me if this has been posted before. I'm in the beginnings of an RSTI build using an '04 ej257 engine and '04 STI ecu in a 2000 RS, but keeping the stock 5 speed. As far as I know, there are two ways I can do this, using stock dash and all interior electronics, but also chop up a $500 harness to merge it. Or I can do a GD dash swap, which I want to do anyways, and just merge the harness for the lights and windows. If any one could clarify for me, or has a better idea as to what my options are, please leave feedback.

Thanks in advance.