4l80e worth it?

[Tdubbyount]

So I've got a 79 k10 shorty, 3/4 ton running gear, mild built 350 and a fresh rebuilt sm465 paired with a 205. I just finished this project and I love every bit of it other than the highway driving! I was told the running gear had 4:10 gears. I'm running 35" tires, and at 70 mph I'm turning 3000 rpm. That gets me about 7 miles to the gallon. Not good. So I'm wondering if it would be worth it it swap out my precious sm465 for an overdrive tranny. I absolutely love having a 4 speed, and I know this transmission will never fail me no matter how hard I beat on it, but I would love to be able to head up to the lakes for the weekend without burning my whole paycheck. This truck gets a lot of off road use, and I tow with it occasionally.
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