I'd do a monster 3.6 to 4.0L, NA, fuel injected ITB motor with a single (stock-ish) exhaust outlet. Lower it, give it the right stance and make it into a dragon slaying sleeper. Add a beefed up, LSD 915.
don't forget to cool it properly. Dual carrera coolers. run the -16 lines through the passenger rocker panel to keep it stock looking.
I'd change the sugar scoops for H4s, maybe yellow lenses.
and on the inside.....I'd do a clean RS carpeting, some retro aftermarket sports seats like those from GTS (I think) maybe with a prototipo.
I might also incorporate some alcantra on the inside, maybe a hand stitched dash cover or headliner. I'm digging the alcantra these days, but use it in a way that maintains the retro sleeper theme.
On the suspension, I'd go stock sport level with 84 to 89 sway bars, TBs and sport shocks. Make it a great touring car setup. I might even update the AC so it could be driven year round.
OK, I just spent about $35k of your money and two years' worth of your free time.
Sound good?
Doug