What are your plans for the car?
Serious question that nobody ever asks...
I don't think 'most' are 3.5" spacing. Most of the one's I've seen have been the 3" M-Caliper setups from early 911's.
Intent - Stockish Engine, Stockish Wheels and tires (meaning no flares and 8 and 9x16" Fuchs) and a HP rating from Stock to 140 = M-Caliper 3" Struts off an early 911T.
With these struts and calipers you'll get: Vented Rotors, Your Choice of Koni, Boge or Bilstein inserts and a slighty larger piston in the caliper. If you bolt on the entire front end you'll get 18.8mm torsion bars in addition (slightly larger than your current bars)
I would recommend a through body sway bar with this setup. I would also recommend a T-Fitting in place of the Proportioning valve for your rear brakes.
Models and Model Years to Look For: 911T, 1969 - 1973
Price Range: $150 - $400 ($150 probably doesn't have calipers, $400 should be a bolt on and go system)