those are the wrong brackets for your truck. those are made for the 2.8 type frame mounts and cant be reversed. if you tried to reverse it, it would hit the serpentine system like you say. even if it could be reversed you would need to buy the 2.8 style mounts to use them. here is a picture of the difference from JTR
your 4.3 uses a clamshell mount that has two pieces, a piece that is mounted to the frame with 3 bolts, and a piece that covers the frame piece (the clamshell) that bolts to the motor.
I havent done it, but others have, where they use the transdapt 4671 flipped and mounted it to the frame instead of the engine, moving the factory frame mount back the max amount the adapter can. I HAVE done it with custom made plates in the exact style of the transdapt 4671 with slightly more length, so I had the 4.3 in my 47 moved back 6 inches and I know it works.