12 slots r cool, on any falcon, centrelines work on xf's good 2 methinks.
As for how low u should go, 1st think if u r going 2 put a v8 in it. Because if u r, dont go 2 low. Or even if u r towing all the time, low cars sux. Or even if u plan 2 do a lot of travelling, with heaps of stuff in the car....... The best thing 2 do would b 2 go c the bloke who's going 2 do it and ask him. That way he has seen your car, and would be able to make a better judgement. U r getting new springs yeah ?
***351c 71 XY Falcon*** Go the Fords !!!