what i was looking to do with my monte was invest in a preformance cat. from jegs, they run about $55 apeice. then run that into a dynomax super- turbo muffler ( $50 some dollars from jegs, i have em on my 65 buick and love 'em)
and just do a stock single pipe, maby with a T leading to dual tips for a cleaner look out back. its not gonna sound like a V8, but that setup should give a monte a nice mellow tone. i would not say to go loud, ( glasspacks / straightpipe / header dumps) thats gonna sound like a... well, any V6 with no exaust, really farty and annoying. plus you would get pulled over alot for not having a proper exaust, and fines would pile up quick, trust me, lol.

 

honestly prob the closest you;ll see for a v8 sound would be a magnaflow cat resonator delete and thrush welded mufflers..they are 30 a piece but really do sound good...