[Cliff R]

The big 455's will absolutely LOVE a 3" head pipe system, X or H and 3" straight thru mufflers. We've moved from 2.5" to 3" on several cars we drive and race here with nice improvements at the track every single time. The "H" is what I use and prefer and it really helps with cancelling out any resonance or "tinny" sounds you often hear with other systems at specific speed/engine RPM's.

At these bigger power levels a set of Ram Air manifolds and 2.5" mandrel bent exhaust with an X or H and straight thru mufflers is the absolute minimum and you are leaving some vehicle performance on the table compared to a good set of headers and larger less restrictive system.

Also be aware that moving to a "drop-base" air cleaner after installing a taller intake or spacers can cancel out more power than you ever hoped to make. You need a minimum of 3" from the top of the carb to the bottom of the air cleaner lid or there may be considerable power loss from that modification.

I'd also add that right on the dyno my modified stock iron intake made 6 more HP than an Edelbrock Performer RPM intake. A modified HO "re-pop" was 4 hp less than the RPM and it was also "modified" same as my iron intake. The test engine was a 440cid (428 based) with 260cfm KRE aluminum "D" ports, Oliver rods, Ross flat tops, 10.6 to 1 compression and custom ground HR cam with Comp XFI lobes (.361") sporting 236/242 @ .050" duration (topped with Crower Enduro 1.6 rockers). It made 497hp with the iron intake, 491hp with the RPM, and dropped to 487hp with the HO intake..........Cliff