I have a 1951 Chrysler crown imperial with the 331 hemi. I recently acquired a 440 big block matched with a 727 torque flight transmission. How hard is it to replace the 331 with the 440?

 

Quit a few mod's to get that one set up, From what I understand.
Welcome to the site from SO-CAL....

 

It's possible that there is a combination of parts to bolt your 440 in, but most likely you will need to fabricate all motor and transmission mounts, drive shaft, shift linkage etc.. You will need to come up with a larger radiator also.

 

I am in the process of this same swap in my 53 Wagon. It isn't a difficult swap but you will need all the parts Iowan suggested plus you will have to swap your power steering with a late model 53 or 54 unit. Motor mounts will have to be fabricated but cross members can be purchased at Summit.
You should consider an 8 3/4 rear end. The 440 is a little much for you stock rear end.
Good luck with you project and let me know if you have any other questions.
Tom (Low Rider)