A rear end from a 1969 Chevy 1/2 ton pickup will fit nicely. You can find these with 3.07 gears. Match that with a T5 transmission with these gears: 4.03 for first, 2.37 for second, 1.49 for third, 1.00 for fourth and .86 for fifth.
Your original 3 speed and a 4.11 rear end will produce these final ratios: 2.94 x 4.11 = 12.08 for first gear. 1.68 x 4.11 = 6.90 for second gear and 1.00 times 4.11 = 4.11 for third. Let's say those are your benchmarks.
Now please watch what happens with the T5 I mentioned and a 3.07 (or 3.08) rear end: 4.03 x 3.07 = 12.37 for first gear. 2.37 x 3.07 = 7.27 for second gear. 1.49 x 3.07 = 4.57 for third gear. 1.00 x 3.07 = 3.07 for fourth gear and .86 x 3.07 = 2.64 for fifth gear. Please look at the comparison...your first 3 speeds are going to be close to the original ratios in the factory 3 speed. What this means quite honestly is that you can still put 1/2 ton of stuff in the bed of your pickup and drive off without burning up your clutch. In other words, your pickup will still drive like it was designed. Your original tires were 6 x 16 and had a 28 inch diameter. If you have slightly larger diameter tires (29 inch, for example) you can multiply each of those final T5/3.07 ratios by .97 (making them 11.99, 7.05, 4.43, 2.98 and 2.56. If you increase the diameter of the back tires please be mindful of your caster angle...you will reduce it unless your front tires are also 29 inches in diameter and reducing that angle will make your truck wander around. At 29 inches diameter some other things in your front end change (scrub point for one) but we're not going into those with this post. Steering geometry is another topic altogether.
Here is the bottom line and the point I'm trying to make: The ratios produced by this transmission and rear end are one way to have your cake and eat it. Your truck will feel just like it did when it was built. Then when you shift to 4th, you'll have a 25% reduction over the 4.11 rear end...this will be your first overdrive gear. And when you shift to fifth, you have your second overdrive. If you're in a very hilly area, you have the option of driving in 4th gear because 5th isn't really that much of a reduction. In my opinion this is a great scheme to solve the gearing challenges in a stock AD truck on today's roads.
Please also pay attention to what Evan (Coilover) has told you. 2000 rpm is your target. With the combination above and a 28 inch diameter tire, you will be turning 2066 at 65 mph. Driving a stock AD truck faster than 65 isn't a brilliant plan. The front end and the brakes simply were not made for speed. Reduce your speed to 63 and you'll be at 2002 rpm. You won't get closer to your target than that. If you have the right carburetor or if your carburetor is in very good condition, you'll be getting the best gas mileage you can ever hope for and your engine won't be lugging. You can use other rear end and transmission ratios to mimic what I've done here, but please sit down with a cup of tea and do your own calculations and please be mindful of the final gear ratios. A high speed rear end mated with a stock transmission in an AD pickup is going to mess your in-town driving up like crazy and you won't be able to haul around a picnic basket in it. If you don't believe what I'm saying about final drive ratios, please go to Google and look up something like Starting Line Ratio. Forget anything you read about a great SLR for a 400 horsepower engine. You don't have one of those. Drill down until you find a great SLR for a six-cylinder pickup. Good luck!
