Isn't the 85460 kit for pre LT1 cars? Kit 85409 would seem to work for LT1 cars
The Thorley site has this listing of application part numbers:
Buick Roadmaster 1991-1993 5.7L 85462
Cadillac Fleetwood 1992-1994 Fleetwood Brougham 5.0L or 5.7L
85460 Chevy Astro 2000-2003 4.3L 85232
Chevy Astro 1999 4.3L and Ground Effects 85226
Chevy Astro 1999 4.3L and Ground Effects 85231
Chevy Astro 1996-1998 4.3L and Ground Effects - Will Not Fit Python Kit 85227
Chevy Astro 1996-1998 4.3L and Ground Effects - Will Not Fit Python Kit 85230
Chevy Astro 1996-1998 4.3L and Xenon “Python” Ground Effects Kit Only 85236
Chevy Camaro 1998-2002 5.7L LS1 85407
Chevy Camaro 1995-1997 5.7L LT1 Equipped with Dual Catalytic Converters 85406
Chevy Camaro 1995 5.7L LT1 Equipped Single Catalytic Converter - Will Not Fit California Vehicles 85405
Chevy Camaro 1993-1994 5.7L LT1 85405
Chevy Camaro 1983-1992 5.0L non High Output (HorE) 85403
Chevy Camaro 1986-1992 Fits California Models 5.0L non Tuned Port Injection High Output (G,F,
85404
Chevy Caprice 1994-1996 5.7L LT1
85409 Chevy Caprice 1991-1993 5.7L 85462
Chevy Impala 1994-1996 Fits SS Models 5.7L LT1
85409 Chevy Impala 1994 5.0L or 5.7L - Does Not Fit the SS Models
85460 Chevy Impala 1992-1993 5.0L or 5.7L
85460 Chevy Monte Carlo 2000-2002 Fits SS Models 3.8L 85410
Chevy Silverado 2002-2005 4.3L, 4.8L or 5.3L Extended Cab Short Bed and Crew Cab Short Bed 85281
Chevy Silverado 1999-2001 4.3L, 4.8L, or 5.3L Extended Cab Short Bed Equipped Stock Single Inlet Muffler 85285
-Joe