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
