i replaced a small block with a 440 in a 72 w100 many, many moons ago and used almost all of the small block wiring harness. i remember running the alternator harness under the intake, and moving the main harness on the firewall around some where it attaches with the fold over tabs. i also used the mount isolators and the chassis mounts from the small block combined with the block mounts from the donor engine [70 new yorker] along with pieces of 3/8 or 1/2" plate to adapt to the small block chassis mounts. the engine was located by connecting the original transmission mount [this had the divorced transfer case], then looking to see what i had to do to connect the engine to the chassis. i even used the new yorker pan with an extended sump made from another pan, as the proper truck pan was not available. it worked out great ! in my application, i was also able to use the hp manifolds from a super bee with the only mod needed being a very slight elongated, shallow "c" cut on the upper frame flange on the passenger side. of course, the pipes were custom bent by an exhaust shop. i know this is way more info than needed, but the wiring harness needed almost nothing changed to be used on the big block.