I would use a 360 before a 400. You can get 360s at the junk yard for $60-120. And other than the driver side mount and converter (or flywheel) balance every thing else off the 318 will work.
65 Belvedere II Station Wagon 69 Coronet R/T convertible 70 GTX 70'cuda 99 Dodge Diesel dually 4X4