How much...

Nobody can give you any useful info until you say what you are trying to end up with. You want to drag race or just a ho-hum motor for commuting? Sounds like you'll probably need another transmission, rear-end, radiator, springs, exhaust work, etc. All that stuff adds up. If you buy a new motor then you probably want to replace the water pump, fuel pump, starter, carb, intake, motor mounts, etc. Then the motor itself could be a lousy $1000 Autozone special or an $7000 go-getter. And you might want to get some stuff for your trans. The trans I just got from a friend didn't come with clutch kit, bellhousing, cable, or motor plate. I'm very well aware of all this because I'm buying all this stuff right now and have been a little surprised at some of the costs I had forgotten to figure in.

Mustang Steve is a great resource:
http://www.mustangsteve.com/351W_Parts.html