Personally I make a point of not trying to convert 40k stats to SC.
Ah ripper swarms - I never really got around to doing much with rippers despite acquiring a good number of the bitey little bastards.
Here's my un-playtested idea of how to handle them in Space Crusade
Move: 8 Squares
Armour: 0
Shooting: -
Hand-to-hand: 1R -- (1 attack per remaining Life Point, stack or divide dice between targets in the same way as shooting the Las-Cannon)
Life Points: Equal to number of Rippers Represented on Base
If you already got them based up for 40k, on a round (or square) 40mm base they should take up 4 squares like the Dreadnought (most people seem to have at least 5-6 per base)
However if you have the option to, mount 3 Rippers on a 25mm square base if you are just using them for Space Crusade and you'll get better tactical flexibility out of them when it comes to movement
You could even attach the little buggers to their base via magnets and remove them as Life Points are lost
-----------------------------------
~The ravings of a single mad Goblin is bad enough, but such a power-hungry, malice-filled creature as Mortis can never hope to be understood~