Anyway, been doing a little research here.
http://horsepowercalculators.net/int...anifold-design
Seems the best method is to bring the pipes into the plenum and stop them above the floor of the plenum. They should also be rounded at the ends to help breathing so I spent a bit time peaning them a bit more by hand.
The dimensions I'm using are probably not ideal but apart from neccesity I'm doing this just to see if I can.
I'm also going to incorporate the idle control valve and that's going to be complicated because the valve lets air into the plenum by bypassing the butterfly and taking it from an opening on the outside of the venturi.