whats the LS swap mpg gains?

[The Big Green]

I have cash to do a LS swap in my 67. I currently have a stock 78ish 350/400t with 882 heads and a RV cam, with a 4.11 gear and 32in tire on rear. I want to be able to take off anywhere with little risk of breaking down and get like 20mpg. What are the best averages made with, 4.8, 5.3, or a TPI setup? I found a 2002 5.3 with 82,000 miles with all that I need to swap for $1100. I need to weigh my options.
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