Fox,
There should be no need to cut the firewall. The V8 is shorter than the six that the truck was designed around. I have seen many 350/400 combos put in AD series trucks without a so much as a grind mark on the firewall/toeboard area. I can't comment on your cross member, but you should be able to set your radiator shell in place and use that as a guide when setting the motor and tranny.
