the 3500 upper will attach to the large-port 3100/3400 lower, so that keeps any 60V6 running a gen3 large-port LIM/UIM open.

if it were 3500 LIM specific, then that would cut out some applications.... the 3500 LIM can only attach to the 3500 heads, which can only be used on 3.4 and 3.5 engines due to bore size/overlap issues on 2.8/3.1 engines. but you probably already knew all of this.