I would caution about going too large on the turbo.
I have DD2's (80hp) and a Smarty. I was running a KSB1-B and it was very laggy. I tow 13k 5er. I lived with it for 4 years, babying it, working at keeping the rpm's up ~ 2000 with downshifts, OD lockouts, etc.
I had it rebuilt to a HTB2 (62/12) and love it. Smaller exhaust, Smaller compressor, way faster spool, actually more boost. I'm back to enjoying towing again (4 years later).
Call HTT and talk to them. They have a good website about sizing the right size turbine to fit your needs. Talk to Brandon and he will help you get a good match for your hp and turbine.
Again, I would caution about going too big on the turbine, not fun working your butt off trying to keep it spooled, and because of the $$ involved you do not want to just "try" that turbine and then decide it is not right, and then "try" another one.
Bob Weis