I’m sure the blank they’re using for their 5/16” stem BBC valves could be made at the correct Hemi length if one really wanted to try it

Of course, for flow testing the length wouldn’t matter.
Just buy the appropriate BBC valve and see how it works.

I had some old Top Alcohol style Hemi heads here a couple years ago.
Some variation of the castings made by Stage V.

2.400 intake valves iirc........ and a normal looking non-tulip design.