Thanks for the comments
I spent a few days lurking, searching, checking FAQ's and so on, so I picked up the brake fluid tip - got plenty of that laying around so I'll get those various parts soaking (got a shedload of WRC dampers too)
As I can't get plasticard (sheet styrene) until the weekend, I've cut a template from card ready.
If you look at a real S13, once the lamps, grille and bumper are removed there isn't a great deal left. There is no steel "valance" to speak of, just the headlamp brackets, and crossmember. There's a prominent washer bottle of some sort on the right hand side, opposite where the SMIC normally sits, so I'll have to replicate that with something translucent, maybe a fogged block of clear plastic.
The radiator hides most of the panel (the largest part in the centre of the template), and in turn the intercooler will hide much of the rad, plus the two horns.
I was going to steal an intercooler from an old Evo kit, but I think I'll make a big one like the GReddy job I saw on a Skyline-engined real S13.
Still undecided on what to use for the intercooler piping, possibly solder wire?