I'm more of a Ford guy, but I've seen BBF's spun higher than that with HR. Same grinder I used, Jay Allen, had done a cam for a 534 in a fox that ran bottom 10's on pump gas, guy drove it to and from the track. Big valves need lots of spring pressure if you are running HR, which likely means you'll be changing them out at some point as they get tired.
The Howards "kits" seem to cap HR at 6,100 for the BBC, whereas the BBF is good for 7,000RPM on an HR setup from the same guys, so there's some merit to the claim that the BBC RPM ceiling may be lower with HR as you've indicated. Did you guys explore what HR might have looked like? Not trying to take this conversation into the weeds, it's just interesting to see an SR setup on a street mill, it's somewhat unusual.