if anything the value will go up with a 340 in it. while it may be neat to see an all original slant duster at a show there is a good reason you don't see many. they are boring as hell to drive. in the end it is all personal preference. its your car so its comes down to what you want. i know we have an original paint 70 dart. slant 6/ auto rubber mat car. i lived with that slob of a motor for as long as i could (about 5 years) and then dropped a 360 in it and its never been more fun to drive.
