Don't use the 6.4 springs get the mopar performance spring set. The individual springs are cheaper though.
www.challengertalk.com
There is a risk that the 6.4 springs in a 5.7 will bind (different install heights). The mopar performance springs are good up to 0.625" lift...more than enough for the 6.4 cam.

Finally an answer to MOPAR performance valve springs
i just got off the phone with a mopar performance parts specalist concerning these performance springs .im going to use them in my 6.4 cam swap in my 5.7 and saving $$. google this P5160074 mopar performance spring kit. perfect for my build . then google this mopar part number 68277306AA. yup ...
There is a risk that the 6.4 springs in a 5.7 will bind (different install heights). The mopar performance springs are good up to 0.625" lift...more than enough for the 6.4 cam.