Not sure about the taper rule of thumb, but here is a good shaped plenum to get you an idea. http://www.ebay.com/itm/2JZGTE-intake-ma...45107da&vxp=mtr

Since you have a "D" shaped plenum, I would only cut a small chamfer like this one.
http://www.ebay.com/itm/2JZ-GE-FFIM-SUPR...686cd19&vxp=mtr


Worrying too much about all these dimensions, plenum taper, velocity stacks even with the plenum floor is really not too much of a concern unless you are really going to try & set a new record.
Same for the turbo exhaust manifold, having individual exhaust header tubes will give you the most amount of power over your SPA manifold, but how much more?

You can have a very, very ugly log type exhaust manifold, that can look like it cannot flow well at all & still make a lot of power.

I had told Douglas to have his fuel injectors installed right in the center of the velocity stacks about 1-2" above the stacks.
He did do part of the idea, but his injectors were a bit further away from the velocity stacks than what I wanted.
If you look at the video, you can see one set of 6 injectors directly over his velocity stacks, IIRC, his other six were placed in the intake runners.

He still made great power & ran w/a blown up engine before the end of the 1/4 mile, & ran a 8.6 @ 165 MPH.

Here is his old car .The yellow Chevy Opala w/a 250 inline Chevy 6.
https://www.youtube.com/watch?v=R825adfxyq8&index=53&list=FLC47WILAMLZIhl7pE8DUelg

MBHD

12 port SDS EFI