Okay, for another opinion (slanted toward being able to still use your pickup as a pickup, enjoy all 5 of those gears usefully and also have nice economy on the highway): Find a rear end from a late 1960s 1/2 ton truck. It will be a 12 bolt and made very well. The width will be almost perfect---within 3/4 inch or so. If you're lucky, you can find a 3.07 ratio. And on another note if you had bought a brand new pickup in 1967, you could have had the 230 six coupled with the 3.07 rear end...it was an option along with 3.73 or 4.11. You could also have gotten the 3.07 in the El Camino...along with the overdrive transmission if you got the 3.73 rear end (and please keep this in mind as I'll hark back in a moment). I've had some people tell me "oh, well matching the 3.07 to an automatic transmission changes everything." Yes, but not in a good way. The automatic had 2 speeds and a 1 to 1 final gear ratio, so if anything it made it worse.
If you have the T5 with the ratio chain that begins with the first gear as 4.03 and ends with fifth gear being .86, you are good to go with a 12 bolt rear of 3.07 and you'll have the 6 bolt hubs. Really? Yes, I've explained this before...maybe 20 times here and equally as many times on other forums. It is a way to make the 5 speed mate with the 235 as if it was designed for it. With your OEM gearing (with the 235) your overall first gear ratio was 12.08 (2.94 x 4.11). This is just perfect if you want to load 800 to 1000 pounds of "stuff" in your bed and drive without problems. And it is likewise ok if you just want to carry a dozen 2 x 4s over to your friend's house. The GM engineers didn't choose this ratio because they were dummies, by the way. If you want an engineer's viewpoint on this from a man who was there at the time, please call Tom Langdon. Please tell him I said "howdy from Texas." He and I have discussed this many times over the years. With T5 first gear of 4.03 and a rear of 3.07, you get a first gear overall of 12.37...two and a half percent off OEM...which is as close as you can get using a T5. Second and third will follow along this same OEM pattern with only fractional differences and when I hit 4th gear I am enjoying a 25% reduction from that annoying and gas guzzling 4.11. And then if I'm on a mildly hilly to flat highway with no headwind problems I pick up another 14% reduction by going to fifth. You do need a 235 made from 1959 forward or at least the camshaft GM designed for those years. And that is because the 1959 through 1962 235 produced higher torque at lower rpm. GM tells why and how they did this in their corporate data and you can read this on their Heritage Pages. Take a look at the torque curve of the 235 from those years while you're at it. At 1800 rpm, you're in spitting distance of making as much torque as you're going to produce and that line stays flat until you almost reach 2700 rpm. It is a beautiful and not very common thing to have a max torque line running flat for nearly 1000 rpm. In fifth, at 65 mph my engine is loping along at about 21~2200 rpm. You mentioned a 3.73. Let's consider a 3.73 rear and a 4.03 first. This gives you an overall first gear ratio of 15.03. That is way more than I like and a factor of 2.66 higher than your truck was designed to have. Which is very notable. Honestly even if you live in a very mountainous area it will tire you rapidly and remind you of a cement mixer. Unless of course you plan to haul a heavy trailer around frequently. You'll be wanting to shift from first to second about halfway through a city intersection. By the way, why did everyone start in 2nd gear with the SM420? Because it had a second gear ratio of 3.57 to 1. 3.57 x 4.11 = 14.67 overall. Even that is more "oomph" than you need for overall first gear unless you're hauling something heavier than a 1/2 ton pickup was designed to do. The difference of a factor of 1+ in your overall first gear ratio matters more than you might think. A factor of 2+ and greater than that is fine if you're wanting to pull barns down or carry 2000 pounds. Not good for driving around in a 1/2 ton pickup, however. Study what GM engineers used in 1/2 ton pickups. As I mentioned, these engineers didn't choose those ratios because they were stupid and they stuck to them for many years when the words "light pickup and inline six" were in the same sentence.
Now, I mentioned the 3 speed overdrive you could have had in 1967. It was a 30% reduction. So let's see what that does for us. Let's say you had the 230 six with a 3.73 rear (a friend of mine had this in an El Camino he bought new) with a 3 speed overdrive. 3.73 x .7 equals a final rear end ratio of 2.61. With mine, I have .86 x 3.07 or a final rear end ratio of 2.64. As for weight the 1967 El Camino weighed 3200 pounds. My 1952 Chevy truck? The same...3200 pounds (actually a bit less because the T5 weighs only 75 pounds). Anyway that's the homework I did on all this...and it works no matter which gear I'm using. The neat thing is if I get into a very hilly to mountainous area, I simply shift back to 4th and I still have the benefit of a 3.07 rear end. Now if I have 1000 pounds in the bed and I find driving on the highway to be taxing on the engine...I simply shift back to 3rd gear and I have an overall ratio of 4.57 (and truthfully closer to 4.11 because I'm using 15 inch wheels). Remember, please with the OEM 3 speed, I would have had 4.11. Literally all bases are covered with this approach and the OEM light truck scheme is preserved as close as it possibly can be. Friends of mine have gone out, bought a T5 without checking the ratio chain at all and a rear end without having any idea whatsoever what it was. Almost every one of them has told me the T5 just didn't work for them. One of them admitted he had a 5 speed with only 3 marginally useful gears.
Now I should add this important gas mileage fact: 55mph is an ideal speed for the AD truck. At any speed over 55, wind drag gets exponentially cuckoo. And it isn't just the AD truck, although the AD isn't exactly heralded for aerodynamic superiority. If I drive our Subaru Forester (which is more efficient than the 1952 Chevy pickup will ever be) at 57 mph on flat to mildly hilly roads without strong headwind, I'll get 40 mpg. If I increase the speed to 65 mph, gas mileage drops to between 30 summer and a max of 32 winter.
My two cents worth. Hope it helps.
