I was told that also when i put the 440 in my cuda..... I got the whole schumacher kit with headers and everything. But after I did mine, a buddy of mine stuck a 440 in his duster and used all stock mounts. He did have to use tti headers tho.
I would recommend a full motor plate instead of the schumacher mounts if you plan on having any kind of power tho. I busted mine really quick and had to go with a solid setup instead of the rubber mounts....