Well, the first attempt at casting the plenum went yesterday and there's good news and bad news.
The bad news...I built the form "floor" out of 1/2 foam, figuring I'd grind/sand/machine the excess away to get what I needed. Unfortunately, the base ended up consuming too much material and the 16 lb pour came up short. I'll be removing material from the floor and from the throttle body mounting flange on version #2. Also, the foam structure was a little weak, so I'll need to add some supports or sprues to minimize warpage and pre-pour distortion.
The good news...it was SO close. Sorry for the crappy cellphone pics, but the pour went very well and it was amazing to see something that I had created in foam reproduced in aluminum.
Also, I thought it was really neat that you can actually see the flow/fill pattern in the side of the casting. All of this is getting covered in black wrinkle paint, but, nonetheless.