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
