One of the engines I'm building this winter for a family member is an oval port 454. We already have the Offy tunnel ram and I'm about to start cutting the top off and drilling the injector ports within the next couple of weeks.
I did a crude mockup tonight using some foamboard and have a basic shape I'm fairly happy with.
I just have two questions to those that have experience with manifolds on the flow bench;

Q1: Should I continue the flat portion of the plenum roof farther towards the back before tapering down? Or will this be okay? I have read before that you should actually extend the plenum a little past the rear ports some with this sort of configuration.

Q2: I have some concern that I may have the Throttlebody (90mm from a Q45) too close to the first pair of runners and high velocity air may pass over the front cylinders and favor the rear a little more. I'm thinking about building the throttlebody another inch or so forward, with a longer radius blend down towards the front runners, if this possibly could pose an issue.

BTW, I already know the foam mockup is REALLY crooked :)

Well I cannot claim flow bench experience, just various things I have read over the years. That said, and lacking any other responses, I will offer my thoughts anyways :D

What you are aiming for is for the tunnel runners to operate as though each was independent. To do this you need to have sufficient volume in the plenum above the runners. I might theorize that you need relatively slower air velocities in the plenum such that they do not allow side affects between cylinders.

So yes I think you need to open up the rear rather than slope it down. And yes I think you need more space between TB and front runners.

I think you may also want to consider more volume to the sides of the plenum.

I also am under the impression that having some space around each runner intake is a good thing (i.e. no walls right up on the runner port). A rounded runner intake "bell" also helps.

I think if the injectors mounted close to the bottom of the Tunnel Ram. You should not have any problem with fuel distrution. Is this how tune port injection works? Just my 2cents.

Yes the injectors will be at the ports. I expect that I'll actually be able to drill the injector holes directly into the manifold without adding bungs, since the manifold is oval port on the inside and still looks like the rectangle port on the outside, its really thick.
Its not fuel distribution I was concerned with, its even air distribution(which ends up having the same effect). If I was doing a downdraft with two throttlebodies I wouldn't worry a bit about it, actually I'm not worried with what I have mocked up either, just wanting to maximize results.
Right now I'm thinking about an inch taller, with the back tapering down closer to the rear. as for the throttlebody, I'm thinking I'll add another inch and a half or so forward and make a long gradual radius down to the front runner group.
Vroom, your analysis is exactly the same as what I was thinking, thats +1 confirmation that I am thinking on the right track. As for the runner openings they look really nice and I plan to leave the lower portion of the plenum alone. In fact once the top is cut off it looks like I wont even have very much smoothing to do down there. Offy really did a nice job on these. I'll post pics of the inside next week when its all cut open.