i need some help with my 72 dodge demon

If I were you I'd look at getting a 360 (roller block, pre mag) la block or a 5.9 magnum motor. 360 are torque makers which is what you want for a street car.
You'll also need to buy a motor mount set (which allows you to bolt a v8 in using a slant 6 kframe) or just buy a v8 k frame (get a 73+ spool mount type). Need a v8 tranny (auto= 904 or 727; or Manual 833 non overdrive) either auto or manual. You'll also need a stronger rearend 8.25 or 8.75 or Dana 60 (the Dana would be kind of over kill for a normal street car with less than 500hp or so). All this is if you want to go to a v8 if not you could always turbo charge that slant 6