I have a 1972 dart custom and i am replacing the original 225 slant six with a mopar 340 v8. the v8 came out of a duster and already has the transmition on it. will i have to buy a new drive shaft because the length is diferent and how easy will the v8 fit back in with the old mounts?  

Malcolm, You can't use the old engine mounts from either engine for this install. The easiest way to do this is to contact Schumacher Creative Services. They sell conversion mounts for this and many other engine swaps. I've used several kits from tehm, and never had a problem. I would plan on shortening the driveshaft.