Do I go twin or stay single scroll
 

Looking into building a 2.1 v2 cdb with v3 heads to about 400-460bhp with a bugeye 6speed short ratio box and r180 diff in my gc8

Turbo I'm thinking of running is either a sc42 or sc46?

Do you think I should go for a single scroll setup like it is now or go for the twin scroll setup?

Not to clued up on the pro's and cons, also any other recommendations?

Its a no brainer really..

experienced both extensively and twin scroll is the way forward

What would the difference in where boost would kick in between the two roughly?

And where could I get a good twin scroll setup kit without turbo for my gc8?

Hard to say conclusively as every setup varies but at a minimum id say 500 rpm but you also have increased torque and better transient boost response. Look at the breaking section for a JDM car breaking and buy the lot from minus turbo, you will need manifold, up pipe, sump, pick up and dip stick

or 2.5 it ;)