There's a balance there, for sure... NASCAR fights hard to keep all brands equal, but that kills the point of having various brands!
Totally 'stock cars' wouldn't be great IMO since we'd lose the safety innovations that have saved so many lives in recent years... Let's instead let manufactures fight for actual dominance like back when the manufacturer championship actually meant something!
On the safety part.
For all the roll cages and such, it takes a harness wrapped around the drivers head to prevent his neck from snapping to save lives.
All other racing has roll cages they can put into existing cars. NASCAR could do the same. Verify that the cage meets standards and go on from there.
Most manufactured cars probably couldn't sustain 160+ MPH races, let alone top 200 MPH.