Not that much of a GM guy but my understanding is that the LS is sever so lightly smaller in most respects than the SBC. But that really depends on what is dressing what.
Tunnel ram on a 350, it'll be taller, but stock intake vs stock intake the LS can be taller. No distributor on the LS so firewall clearance is better on an LS. Exhaust ports sit higher on an LS, so header clearances can be different. Oil pan choices are slimmer on an LS and not as easily cut and welded to clear. Not really a clear answer for you.
https://paceperformance.com/p-3990-engine-dimensions.htmlThe GenIII and various other Mopar engine dimensions
http://st.hotrod.com/uploads/sites/21/2016/10/54-Chrysler-engine-dimensionstable.jpg
