Motor plates take place of regular motor mounts,so all that area is left clean and headers dont have to fit around them.
But,motor plates are almost never used on street type cars,they tie the motor real tight to the chassis and transfer alot of shakes and vibrations in the process.
They make front and mid motor plates,the front mounts right throught the water pump area,and the mid mounts between the trans and motor.
If you need some weird header combo made check out Tubular Automotive,I had some made from them for a Boss 302 with Aussie 302-C heads in a 65 Falcon,they had not trouble making the headers fit and work perfect.
I remember that the catalog they had listed some Granada headers in it.