if you put in a 400 you are going to have to change the eng mounting as you will loose your bellhousing mounting and will require side mounting for the eng
if you could find one and if you wanted an a/t with that motor which would bolt up and also be able to side mount, would be to put in a hydramatic. 4spd and same final drive
http://yfrog.com/12dscn2074lj
ron