Were your engines dynoed that way on the chasis and/or engine dynomometer ? My engine combo falls inline with your reference and I plan to keep the 1.60 Exhaust size. Yes on the shrouding and I believe the Sunnen VGS-20 will do an adequate job here clearing metal away.

Were your heads ported by hand or CNC? Who did your valve-job, did you stick to a 45* seat and 30* Back-cut?

There are many determining factors to gain HP and I consider many things involved in both Bottom and Top-end as contributors.

The size of the valve may or may not be the most important factor to power but since it is in direct contact to flow, I believe in this case, "size matters" along with the diameter of the valve stem, quality of the valve job and even weight of the valve.

Does this all matter in building a hot-rod engine? Maybe or marbe not but if I do find myself at 1/4 mile track and test the limits of this engine, every decision I make now will show itself on the track.