You can get an SD to about 3.25 Square inches (2.375 x 1.375) and maintain good taper to the plenum and it works really well. A standard MW port is (IIRC) 2.6" x 1.3" and a standard 906 is around 2.85 Sq in (I forget the exact dimensions but it's like 2.27 x 1.23...ish?) My SD plenum porting (only) was ~.15 better in ET and 1+ mph on a 493" in a 3700+ pound high 10 B body with Standard port E heads. In order to max out he runner the ports in the head need to be opened up a bit as well, but the key is the intake should not reduce the flow of the port, as many, most do...at least in the powerband/rpm range you're tuning to.

IMO, The issue with the SD and many other single planes is the consecutive cylinder firing between the 2-1 and 5-7 pulses where those runners happen to be adjacent. the 'first pig at the trough' (in this case the plenum) tends to steal a little from the second. Dual planes have other distribution issues, just not THAT particular issue.

I love a Ported SD on a 500", but a ported Indy Dual plane might be the way I'd go on that one, open up and radius those lower plenum to runner transitions, particularly # 7. I could see how there could be an A/F distribution problem with an As Cast 440-2d MW manifold...the uppers flow about 355-360 but the lowers only do about 320...again AS CAST.....so doesn't mean it's a bad manifold, just like a Victor 340 or a BBM SD, the potential from porting really is there...and it's well worth the effort to go get.