How much fuel should I save ?

[1984 t type]

I have a 91 jimmy with a tbi 350 and it threw a rod. Thinking about doing a 5.3 swap instead of putting in another used 350 but curious as to how much fuel I will actually save by doing the swap. Will be a daily drive that will not be going off road or doing much if any towing. Looks like I can pick up a good 350 for about 500 bucks, and the 5.3's are running about 600-800 complete with all the accessories but I would still have at least another grand tied up in a harness and all of the other supporting mods. Guess I'm kinda at a lost on if it would be worth it for a truck I may only keep a few years. Also this won't be my first LS swap so I'm aware of what's all involved and capable of doing all work but the harness and PCM flash myself. Just looking for you guys thoughts and input on this one. Thanks
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