For a numbers matching car I usually add (on top of a non-#'s car) about $10,000 for 340's, 360's and 383's, $12,000+ for 440's and $15,000+ for 440-SIXPAKS and $25,000+ for HEMIs. The R/T cars (383ci +) add the full $10,000 IMO and usually plus dependent on options and rarity.
I doesn't really matter too much, I own a #'s 70 Challenger R/T and a clone 340ci 73 'Cuda.
I love both!! At the end of the day it makes no difference to how they drive, does it?
They're just #'s.
Get what you want, if your after something original and for the show scene a #'s car will do you well.
If you're after something to drive more and enjoy and not worry about so much a non-#'s maybe your best choice.
Hope this helps.