The 1/24 Monogram/Revell '70 Chevelle/Malibu is probably the best kit, sad to say.
For the ultimate '70 Chevelle SS, combine the body parts (and maybe the interior) from the 1/25 Revell SnapTite '70 Chevelle SS 454 with the chassis from the 1/25 Revell '72 Olds Cutlass kit, and the BBC engine from the AMT '70 Monte Carlo SS454. The Monte has a very nice set of stock exhaust manifolds, but you will nee to source the correct air cleaner elsewhere.
You think the snap body and interior are better then the 3n 1 kit? I would rather get the 3n1for the extra parts value etc. I was mainly just disappointed with the undercarriage and engine/engine bay details.