stock magnums flow okay, but they have their limits. if you're going to port something stock the mags have quite a bit more upside than stock LA heads. but then you're talking about the incurred cost of heads, rockers and that work. the other side of that coin is some aftermarket heads which should (in theory) flow at least or better than magnums all across the board. so there's that?
i'm gonna use some very loose numbers here, but on that particular 400hp build the magnum RT heads used have an intake runner of 180 and flow about 220-230 out of the box, so a wild *** scientific guess would say you need "about that", so there would be my target range for deciding aftermarket heads. well, that and rocker gear-- i don't know if you've got something decent you can reuse or if you wanted to upgrade.
if you can get away with a polish, then i'd just slam bearings in and ship it. but if it's beyond that or needs another cut, then it's decision time. i can't wholly make a recommendation because i don't know what availability is like over yonder, but if you've got a crank requiring work and those bores hold unexpected surprises then it may be time to rethink the path forward, especially with the block already being 40 over.
babies n' bath water n' all that.
that's just the way the scone crumbles homie. set out to do it one way and the script gets flipped on ya. we fully expect an update on the bores and i look forward to help spending your money for you!