Personally think if you've done some small engine mods then best way is to use a O2 wideband sensor and gauge to get the correct fuel mixture, takes the guess work out. I would start with the standard needle for the engine and ensure fuel ratio is set correctly at idle as per Volvos instructions. You can then plot the needles here for comparison to help chose:
http://www.mintylamb.co.uk/suneedle/
For SUs I would shoot for the following air fuel ratios, they will vary a bit with the SUs when looking at the gauge:
Idle - 13.2:1
Cruise 14 to 15:1
WOT 12.5:1