one reason why you want a significant volume in your plenum is because as each cylinder draws, the airflow through the throttle
cannot keep up with the demand, thus pressure drops, and the cylinder sees less boost as the induction cycle moves towards its end.
that 2x2 box is sooooo small.
if i were you, i would consider chopping the runners off a lot shorter and putting a large flat box over all of them rather than between them like that.
if you cant access the bolts like that you could tig in bolt access tubes and or use long bolts to do it from the top all the way through.
as bad as the low volume is the sharp edges... how are you going to properly radius the ends of those runners? ideally you want a bell mouth in there, but even a slight flare would be better than a sharp square edge. even just having the runners protrude into the box a bit will be better than a T joint.
also with such a terribly small volume, its highly unlikely that the far end runners will see the same flow as the near end ones. that is one area where you must be very very careful if you dont want to run into one cylinder being a bunch leaner than some other one. this is a real problem. and a serious one too.
heres a thread on how i built mine for my 2.0, its i would say over 3.0 in volume, and has nice radiused entries to the runners. its not ideal though, as proper bell mouths with turned back edges would have been best, but its pretty damn good. not the distance from the number 4 runner to the back wall, and the tapered nature of it to ensure even air distribution.
The 2 bagel intake manifold.
i appreciate you have limited space, but it would be a real shame to put in a LOT of work and end up with something worse.
i just had another thought.
if you put flanges on the tops of the runners and build it in four parts, a bottom section in a U shape, a top cover plate, and two stub port adapters, you could install them first, put gaskets or rtv, bolt down the U shaped piece, and then clean up the insides if you use rtv, then install the top plate.
considering the amount of power you can make on a stock setup, what power are you aiming for here? whatever your goals, you need to be very careful indeed to get this right or it wont be pretty in the end.
fred.