1970 barracuda gran coupe advice?

So me and my dad got a 1970 barracuda gran coupe, it has all original parts but needs to be restored, it's got matching numbers 318 and auto trans in it that needs a rebuild and my question is me and my dad have been going back and fourth on whether to rip the 318 and auto trans out resto mod it and put a 440 or a 425 hemi or even a 340 with a manual but if we were talking about what would get more money, would it be better to keep as much of the car original with the original 318 and auto trans or put new engine with manual trans? It's also a original ac car with original blue paint that needs to be repainted but my main question is what would get the most money at a auction? Original with 318 and auto or new engine with manual? Thanks a bunch!