If you are planning to put that engine and trans in a '67 keep in mind that the two-cam 4.6 is nearly as wide as the four-cam.  I did a quickie measurement on the engine in my 97 F150 and it was nearly, or at 36" close as I could tell.

Just sayin'  as you may not want to remove the towers, etc.

Other than that, as LC says....get everything.  The harness can be modified easier than making a new one and there won't be enough difference between the Marquis and a Mustang to make the needed mods significantly easier...or harder.  Be sure to get the Relay Control box, has a big R on it probably and will be bolted to the core support of inner fender near the core support.  And the computer for sure even though you will have to mess with the software by use of a Moats Quarterhorse or similar add-on.    Heck, just get every thing north of the firewall that the engine harness connects to. You will strip out all the ABS and Air bag sensor wiring and a bunch of other unwanted stuff but its easier to start with the whole works. 

When I stripped the 89 TC for our HT I took all four door lock motors and trunk latch actuator and the power antenna.  All this stuff is usable or tradeable.

BB



BB

Thanks for all comments everyone.  After considering the cost to make the 4.6 fit and mods to the suspension /shock towers, I think I'll just put a 351 in it.