if you like big numbers and are gonna want big twin numbers i would go with a 64mm secondary, somethin like a 64/71/13-14, then put an s480 under it later, that would be good for lots or ponies and have better street manors than you have now.
If ya want to do twins my braddah, then you should look into a 62 over a S475 for the cheap fun route, can make a good bit of power and spool quick and have good top end. You could go alot smaller than your SB and still make the same power.