Intake manifold questions

It's got the small 273/318 port size, so it would be silly to run it on a 340 or 360.
It's actually not too bad of a manifold for a 273, makes some good torque on the little motor and breaths well enough to feed a nice little motor- but it's gonna run out of steam real quick with even a healthy 'teener (an MPG build would be something else, though...).
You can't open up the plenum very much since the outside of the manifold also necks down to follow that bottleneck; opening it up would blow right through the casting.