Port-o-sonic or Holley Street Dominator?

Holley Streetmaster? Can I assume you mean the Street Dominator? Or do you mean the Edelbrock Streetmaster? The Street Dominator isn't bad, the Streetmaster is.

The old "single plane v. dual plane" discussion is far less important where MPFI is concerned, unless you're really interested port speed for bottom-end grunt. The Street Dominator has smallish, slightly-longer ports than the Offy, or say an Edebrock Victor or Strip Dominator. I think it would be good choice for an MPFI intake for that reason, along with relatively low overall height and ease of consistent injector height... although the same goes for the Port-O-Sonic in terms of the latter two. The Dominator would be more torque-oriented, but the difference would likiley be virtually negligible.

If the other choice is the Eddy Streetmaster, it's Offy hands down. The only aftermarket intake arguably worse than the Streetmaster is Edelbrock's SP2P. Either might work well on a mild 273 or 318, but are pretty restrictive for the 340/360 engines.

Just a word of caution: The factory ECM can't account for increases in airflow through the engine such as hotter cams, ported heads, etc. It sounds like you're staying pretty mild on the cam, but it's worth mentioning. A couple of acquaintances who believed that "the oxygen sensor will figure it out" got expensive lessons regarding the limitations of speed-density EFI.