Howdy Tomtron and all:
I put a '72 250 with a three speed into my '65 Ranchero. It replaced a 200. The 250 is wider at the bottom of the block, just shy of 2" taller and about an inch longer due to the difference in water pumps. The 250 radiator hoses are 1.75". The 200s are 1.5" I used the stock radiator and used hose adaptors to fit the larger hoses to the smaller radiator bunges.
*compare the oil pan profiles. If they are the same, no problem. If they are different assess what might interfer. 250 oil pans are not interchangeable with 170s or 200s. Early Maverick/Comets used an oil pan that is compatible with the '64/'65 Falcons. I don't know about a '78 Granada pan. John or Adam may know.
*You will have to adapt/make a trans mount.
*Use the front yolk of the C4 drive shaft to fit into the trans.
*Slot the engine mounts to allow the engine to sit as far back and low as possible. Carefully compare the engine mounts from both vehicles and use the combo that has the engine sitting as low and as far back as possible.
*I moved the radiator forward by cutting access slots in the top of the radiator support and mounting the radiator in front of it.
*hood clearance to the air cleaner will be very close.
*About 40 lbs of additional weight will put extra load on your front springs.
You would do well to get the Trans mount cross member from the Granada to adapt or use as a template in making a new one. Also get the drive shaft for the front yolk.
I think the 250 C4 swap will make a world of difference in your '64 vert. 80 more cubes and another gear? Whoa.... Is it worth the effort? Your call. Clearly, a 200/C4 would be less hassle, but you'd still have to deal with the trans mount/driveshaft and possibly the radiator, as late model 200s have a longer water pump.
Adios, David