Dishing flip flop or spacers behind sprockets?

option 2.
1.5mm of cog spacers should'nt be a problem (you can use BB spacers), just make sure the lockring still threads on nicely.
http://www.velosolo.co.uk/shoptrack.html

(ED: I've got 2x1mm spacers on my fixed, and the lockring still does enough turns)

Don't forget the 0.01mm inverse spacer to take it down to 1.49mm