No need to pop the rear gear cover. I can tell you, if you've given accurate info, as in if your tach and speedo are accurate, you're running a 4.10 rear gear ratio. OE tire size is probably 31in tire, which isn't quite 31 inch. 3300rpm @ 70mph is 3110 rpm and 200-300 for Torque Converter loss since Th400 does NOT lock up and puts your #'s right on target for 4.10's.
Yes, overdrive would be nice, and yes 700r4 would be nice IF, it were strong enough for a 3/4 ton. Do realize half tons did get the 700r4. 3/4 tons would have gotten it too had GM thought the 700r4 was tough enough. Even with a good build, it's not likely to last as long as an OE built Th400.
Here are your 5 Options and pretty easy to narrow down to 2 and take your pick based on your actual usage of the vehicle and funds you're willing to spend keeoing in mind what's feasible.
1) Gear Vendor add on Overdrive/Underdrive unit. The Unit is expensive, plus cutting of driveshafts, and crossmember mods to make it work. Usually so expensive, unless you're putting 40,000-50,000 miles a year on your Burb and it being able to pay for itself in about 4-5 years in gas savings, where as with many of us, our squares are secondary or recreational use vehicles and might see 10,000 miles a year, it would take over 10 years to pay for itself.
2) Half Ton 700r4 Overdrive Trans Conversion. Even if I were to build my own for a 3/4 ton, to buy all the good strong super hold servo's, Sonnax aftermarket strengthened hard parts, like the Beast or smart shell, input shaft sleeve mod, roller clutches/sorags etc, valve body and boost valve mods, to make it even worth giving a try which I intend to do someday, would cost close to $1000 in parts. That's with me building it and I woudln't give myself a warranty on it if I were using it for any kind of towing or off-road abuse. It might hold up for awhile, it might not. There will always be the weak link in a 700r4. You'd spend close to $2000 to have a Box Shop, Like Monster Trans or Mad Dog to build you one and with shipping it would be over $2000 and I doubt they warranty it too and it wouldn't have all the good parts I'd have in one I built. Just a standard OE built 700r4 that you could get for $1000 or so, it'll work but for how long?
3) Gear Ratio Change. Could change to rear gears to 3.73 but that will only drop you down about 350 rpm @ 70mph and lose a tad of your bottom end take off or trailer pulling grunt and no real mpg savings at all. Also have the option of dropping to a 3.42 ratio if you're running a 14b Semi Float Rear Axle, or 3.21 if running a 14b Full Float Axle. the 3.42 might be OK if you're not towing, but 3.21 which I have in a 3/4 ton pick up with a 454/Th400 and is a sled as far as I'm concerned. With a gear swap, if you pay to have if you Cash and Carry your axle to a shop to have it done, you're probably looking at $800-$1100 to have it set up with all new bearings, seals and gears. And no matter what gear you run, you're giving up something to have something else. Giving up lower end towing power to gain a few rpm reduction and very little gain if any at all in mpg.
Another Option to this Option.. You could see about finding a used complete rear axle, drum to drum either in 3.73 or 3.42 and give up on finding a 3.21. They're so rare you'd be like winning a power ball lottery to find one. You can usually find one of these used axles for $200-$300 and bolt it in. Personally, I think I'd look at another option since you're giving up something to gain something here.
The Better of Both World Options !!!
4) The manual trans NV4500 Option. Pretty straight forward, just as it says. Convert to the manual NV4500 5 speed with Overdrive transmission used in 3/4 and 1 ton trucks. Keeping your low 4.10 gears for the low end take off and towing power and compensating the low rear gear ratio with an Overdrive of .73. In the same scenario, 4.10 rear rear on a 31in OE tire size @ 70mph drops your rpms from 3300 down to 2300rpm and you'll actually get it since it's a clutch and not a slipping non locking TC. The NV4500 is rated at 14,500 GVWR. You may actually see an improvement in mpg here and it eventually pay for itself. You haven't said if this is a 2wd or 4wd Burb. 2wd conversion will be much less than a 4x4 conversion and cost would depend on how cheap you found your used NV4500 for along with bell housing, clutch pedal assembly, clutch and driveshaft mod. For 2wd, it could possibly be done for under $1000 and no electrnoics to deal with along with having a damn near bullet proof trans that can tolerate about all the abuse you can throw at it.
5) The 4L80-E Auto Overdrive as Jim suggested. All the same advantages as option #4 except in an auto trans with a lock up Torque Converter and .75 Overdrive. Being the converter locks up, same scenario with 4.10 rear gear on 31in tires @ 70mph, you're turning from 3300rpm down to 2350 rpm. You will have to deal with electonics to do this swap. Being that you're running an ECM already anyway, and the 91 3/4 ton Burb ran a 4L80-E OE, as Jim suggested, run that ECM with the harness in place of the ECM you're running now, otherwise you'd have to get an aftermarket controller which have now come down in price over the last few years to about $700 just for a controller. Best to find the OE ECM in a salvage yard and go that route. If you could acquire the ECM, Harness, Used Trans and converter, you could probably get this done for under $1000 too. And same as above, hopefully a bit of highway mpg gain so the swap can eventually pay for itself.
Myself, I'd consider option 4 or 5 and have better of both worlds with low rear gear for towing and overdrive to compensate it for highway driving.
