i like the 440 idea.a 383 would be a good one also. sumit and jegs used to have a 505 wedge listed as a crate motor before i left, that would be gobs of power for a valiant. k members arent to hard to modify. just make sure it is strong (1/4" deck plate to reinforce the remaining metal)
check what rearend it has. a dana 60 would be a godsend, but if it was a slant 6 originaly, it wont have one. the 8 3/4" rear is plenty strong for street use. also a 8 3/4" rear wont set you back near as much in the cash dept.
if you want, look me up when i get back to the states, i have some books on mopar you can read through.