Melzi with discrete components.

Hi, I'm new here. I want to redesign the Melzi PCB for using with discrete components, and for using with ATMEGA 644P, all these requirements are because in our country there's a lot of importing regulation and limitations.
We are a group of Reprap users that we want to upgrade to Melzi (near all of us, uses Sanguinololu, and some uses, Generation 6).

Question, as far as I read ATMEGA644P or ATMEGA1284P, are nearly exactly the same is that correct. ? (only differenece is that 1284P has 16bit T/C 3 function and 644P don't) the reason of this question is that here we don't have acces to 1284P, instead 644P is avalaible in DIL40.


Thanks very much.
Regards