There is already a precedent for SM basilisks. The Iron Warriors use them in the Chaos list, and they have the same stats and cost as an IG basilisk. If you are showing a marine crew, you could go ahead and upgrade the basilisk to BS4 if you like, but it will have no effect on the game whatsoever.
If you are set on doing this, I would just use a standard basilisk. The basilisk is good enough as it is, and does not need a further upgrade. I would recommend against doing this at all, however. Adding heavy artillery to marine firepower lists can make them horrendously cheesy and no fun for your opponents to play against.
Basilisks are lousy on CoD due to the small table size (almost nothing will be within the minimum range for indirect fire), poor LoS, lots of cover to negate the AP3, and the special rules for city ruins terrain that can reduce the impact of ordinance, and can negate barrage weapons entirely.