Sorry. I would have to disagree.
I think this is a case where logistics outweighs economics.
With 4,000 - 5,000 vehicles showing up at the bigger NSRA events there are very few venues that can accommodate the shows as is. Opening the events to post-48 vehicles would only make it more difficult to find a large enough venue.
With many new people getting into street rodding, and MANY new pre-49 cars under construction, I don't see participation in the NSRA events falling off.
I'm probably wrong here, but I thought they had a limited registration anyway.?