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I have a 1969 Chevy c10 with a 6 cyl 3 speed. How do I gear this to be able to go 70 mph without the rpm's s high?

What's the present rear end ratio? Are you building a truck, or a Cowboy Cadillac that will never see a heavier load than a Yeti cooler? If you're planning on doing real truck stuff, you've probably got the correct gear now.
Jerry

By changing the rear end for 70 on the highway, you will not be able to get out of your own way under a load in first gear. Been there, done that and they didn't give out any T-shirts.
You would be better off with a transmission with overdrive.
Carl

Explore this web sight (below) this fellow knows the T5 transmission well. you'l probably need to swap your rear end also.

https://lugnutz65chevystepside.weebly.com/thinking-about-installing-a-t5-read-this-first.html

I run a 3.08 gear with 235/75R15 radials, ( 28" tall tire ). 250 ci engine, T-350 automatic transmission with 1-1 final drive. At 70 miles per hour the engine is running 2550 - 2600 rpm. Around town is just fine, truck weighs 3150 lbs so probably lighter then yours. I had a 3.42 for awhile, and yes it was quicker from a dead stop, but highway driving was a pain due to high rpm ( 3000 rpm ) and the constant attention to maintaining speed. At much higher rpm, even the littlest throttle change would speed up or slow down the truck, once I got the rpm down with the gear change, it is much easier to maintain a constant speed.
Calculator: https://www.randysworldwide.com/calculators/rpm/

If you have a 1969 C10 it is likely you have a 12 bolt rear end and if the engine is running really fast at 70 you could have the 4.11 rear end (and maybe the much less common 4.56), however I think 3.73 was stock with the six. The good news is you could lower this rear end ratio to 3.08 if you wanted to...same ratio as Joe above has. I think I'd start by figuring out my starting line ratio currently. You will need to know your first gear ratio (which I think is 2.94) and your rear end ratio and this isn't difficult to figure out. With a 6 and a 3 speed, I'm going to suspect it would be ~11 if you have the 3.73 and ~12 if you have the 4.11. Some will tell you an slr of 10 will work but in a pickup with a six it will feel very puny in town. If you like the way the truck currently drives in town, you'll want to stay with a starting line ratio of about what you have. A quick and easy way to estimate the slr is rear end ratio x first gear ratio. Then if your rear tire diameter is greater than the stock tire, this will result in a lower slr and if your rear tire is less than the stock tire's diameter your slr will be higher. There are lots of ways to tweak this.

For sake of ease, let's use an slr of 12. With a 1969 C10 and a six this should be very driveable...especially if you want power in the low gears and most especially if you want to haul anything or pull a trailer. If you buy a T5 transmission with a 4.03 first gear and you change your rear end to 3.08 you've got just what you need as a starting point. Your basic slr will be 4.03 x 3.08 or around 12.4. Rear tires with a diameter 10% lower than your current tires will bring this down to around 11.3 or so. If your T5 has this first gear, it is very likely its second gear will be a 2.37 ratio. Third will be 1.49 and fourth will be 1.00 so in fourth gear with a lower geared rear end I believe it is a guarantee your engine will be running at lower rpms than it is today. So if you're following my drift, you'll see 4th gear is really going to be like your first overdrive gear. Now when you get into 5th gear, you'll want the T5 with a ratio .86 and this will reduce your rear end ratio to 2.65 or so and you'll be zipping along at about 2200 rpms. Taller tires in the rear will lower those rpms, but please remember taller tires in the back and lower ones in the front will affect your front end geometry...
Jon

Oh one other thing I forgot to mention. If you go with a T5, you may be able to use your old driveshaft, but you will need a different tail shaft.
Jon

Borgwarner 3sp overdrive transmission was offered up through 1972 so the solution is both cheap and easy. This gives a deep gear for starting off or load hauling but a shallow gear for highway cruising. Would be better if a 4.56 as the final would be 3.20 in OD which is a good cruising gear while the 4.11 is a bit shallow at 2.88 in OD. A 57 3100 we built with a 235 and OD was fine on level highways which covers most of Texas but not a good hill climber as the factory gear in a 57 is 3.90 which gives a 2.73 final in OD. Factory bolt on parts can fix your problem.

Remember this also, low rpm doesn't always mean better fuel mileage, if that is what you are after. If the engine doesn't have the torque required at highway speed, you will be in the throttle a lot deeper then you think just to maintain speed. As you are calculating gears, figure in the sweet spot ( rpm ) the engine likes to run at. My 250 likes 2200 to 2600, it will cruise all day without any down shifting or much throttle.

When you are talking about a manual starting out with a load in first gear, automatics are different because you have a torque converter, but none the less good info there.
A vacuum gauge can help you determine fuel efficiency. Like another has said a too tall a gear just to get your low cruising rpm might not be the most economical.