LS transmission questions and other Ls stuff.

[Awann99]

So I’ve finally decided to begin the process of slowly piecing together everything I need to LS Swap 1969 Lwb gmc 1500. I drive 66 miles round trip 5 days a week. Most of it is highway driving and the 350/350 combo getting 11 mpg is cutting it. Fuel economy is my main goal in this swap so I was thinking a 5.3 mated to a 6L80e. Double overdrive = better mpg in theory. I already know I will have to install a big jump in the truck for the transmission to fit. I would keep everything stock and get the stock harness rewired. I’m not look for preformance. My truck has a stock 308 rear ended and I just got a parts truck with a 354 rear end. What rear end would help keep the rpms lower? I’m swapping discs onto the truck as well. I am most likely going to find a junkyard combo because it’s cheaper. My question is what are ways to make this things extremely efficient. I’m not worried about power. I won’t be pulling anything. I just want a good daily driver. Will a older 5.3 mate up to a 6l80e? Please give me your advice.
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