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I try to avoid the truck harnesses as much as possible due to the odd layout, they are harder to make look nice and they are wired a little more complexly than the car ones (more keyed power sources) Here is where you have to decide what direction you are going to go, plan on leaving the less attractive truck intake (still make good power) or switch to the car style intake/ rails/ injectors to make it look more like an actual LS1.
A stock harness usually will run around 0-200 dollars and you will then have to spend 3-4 hours cutting it apart/ figuring out the wires that won't be needed for your application and removing them and finding the wires you will need to connect to make your swap run.
These two factors do not warrant the big difference in price to me.
You can pull your factory sending unit and find a way to mount the pump to the sender and install in tank, or maybe even an aftermarket vendor makes a sending unit for your application to do just that.
Other options including welding a sump to the bottom of the tank and feeding from there.
It would take a lot of work to modify every wire on an older harness to make it layout as nicely as one of the custom built from scratch harnesses.
The next item to consider is what transmission you plan on running.
They may gag, puke, cry or even quit in the middle of the action.