For power, SSI's and a good sport muffler are your best bet. While your at it, ditch the smog pump (which you won't need/can't use anymore) and the electric boost fan for the heater system (heat will still work).
Remember you need the correct oil lines.
Also for your reading:
Pelican Technical Article: Installing SSI Heat Exchangers on your 911
I wish I could tell you how much they help... my SSIs and an M&K go on next week
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Alternatively, Bilsteins and Turbo Tie Rods will do wonders for the handling. This was the 2nd thing I did with my car (1st was brakes which was more of a rebuild/replace than an upgrade), and boy what a difference.
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Don't Lift... Don't Lift... Don't Lift
'75 Targa in "Arrest Me" Red, 3.0SC ('79) engine, Bilsteins, Turbo Tie-rods, SSIs into 2-1 M&K muffler... and looking for my next upgrade.