My boss's LBZ is making about 600 or a little over with just the twin kit, AirDogII, built trans and an EFI tune... unfortunately he just cracked a piston in it soooooooo if you don't have a built engine beware.
I've been running MPI twins on my street/tow truck for almost 4 years of major abuse and no problems with stock engine. I tune somewhere in the neighborhood of 50 twin trucks with stock engines without failure.
Big boost and twins need good tuning to live a happy life.
LBZ/LMM pistons are a toss up, but on average work really well with twins.