A lot of people have cut and shut RB manifolds. maybe one or two have done it in such a way for ALL the runners to flow correctly. most fail at it and at best starve no.s 5 & 6 while the rest run well enough.
the same guys usually just throw a vl rb30 bottom end under it,boost it up and hope for the best. if it works at all,that's good enough. working well costs money,and we don't want to spend money after all...
if you're contemplating this,it'd be best to at least use a trust manifold,it'll be more likely to almost work properly,as the design will doubtless have been tested properly.
ideally you'd use RB26 throttles,plenum and an adaptor manifold,along with programmable management.
Justin...