258cfm would be hellacious exhaust flow for a "street oriented" FE head. Just going by some modern FE castings, they're doing well to achieve 72% exhaust to intake flow ratio (using some posted figures for BBMs and Felony)
If it flowed 258cfm, the intake would be flowing in the 360cfm using the 72% ratio as "rule of thumb". Don't see that happening with a 175cc intake port. Of course, they could be skewed with a really good, slightly raised exhaust port, and hit an 80% ratio, so call it 325cfm on the intake side if that were the case, I see that as "maybe" possible with a good CNC intake port but have my doubts it can happen at .500" with a small port. The Colvert/Kuntz CNC BBM heads go 321/226 at .500", but they're more in the area of 195cc intake volume.
It's all conjecture at this point, obviously, just thinking out loud.