Actually, your existing motor mounts will work- - - -you'll just need to move the passenger's side mount back to align with the driver's side. There's usually a set of holes drilled in the frame for the V-8 mount, as it's in the same place as the one the 250 6-cylinder uses. If not, just measure carefully, and drill and bolt the mount bracket into place.

The only reason to use headers instead of exhaust manifolds would be if you own stock in a company that manufactures spark plug wires. Headers are virtually impossible to keep tight, and the exhaust leaks eat wires in a hurry!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!