I think a very safe answer to that question is NO!
from every piece of documentation I can get my hands on, there was no LS-6 offered in Monte Carlos in 70.
It was however an available option in 71, although no one to my knowlege has ever produced any document to prove that one of those exists either.
The closest I've seen/heard of is one Dick Harrell converted SS454 Monte to an LS6.
take a look
here
__________________
Dave C.
68 SS396 Camaro
69 SS396 CB Chevelle Conv.
70 340 Dart Swinger
70 SS396 Chevelle 4spd
70 SS396 Chevelle
70 BB 4spd Monte Carlo
70 SS454 Monte Carlo
71 SB 4spd Monte Carlo
71 SS454 Monte Carlo
71 Corvette LS-5 conv.
71 Corvette LT-1 conv.
71 Corvette LS-6 coupe
72 Monte Carlo
72 Monte Carlo Custom
72 442 W-30 clone
76 Trans Am SE 455 4 spd
85 Mustang GT Conv.
86 Grand National
02 SS conv.
