ok, doesn't matter on the 6, 7 or 8 speed as there's virtually no difference. spocalc gives these lengths for your current wheels (36h, x3) (actual spoke length in brackets)
front 296.1 (296)
rear DS 293.2 (293)
rear NDS 294.9 (295)
so more or less what you have now.
changing the ERD from 608 to 606 gives the following:
front 295.1 (295)
rear DS 292.2 (292)
rear NDS 293.9 (294)
The DT calculator gives the same results.
So if all the spokes from your rear wheel are the same length at 295mm (really they shouldn't be, but it's not that unusual) and they're in good condition, then you could use them on the front and you'll only need new spokes on the rear. The spokes you have all seem a little long for the old build, if they were on the short side you might get away with using the exact same spokes all round. If you like, I'll have a dig through the spokes I have and see if I can find some 292 and 294 lengths or very close to.
Andy