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A sm465 will bolt right in without any issue using parts from another truck and bolting everything right up for the most part. I am actually weird and like the direct feel of a mechanical linkage. But, if you want hydraulic post 84 trucks have it.

The sm465 is a 4 speed toploader and most of the time you will only use 3 speeds. The gap between 2nd and 3rd is quite sever and 4th is a direct ratio. There are a few issues with the shifter popping out of gear but it is a easy fix and well documented.

That being said it is a very tough transmission and take tons of abuse. This is a dump truck transmission don't expect to be slap shifting this thing. People have put great amounts of torque (500+ft/lbs) through these without concern, ran them with little fluid, and other crazy things and the trans just laughs and keeps going.
In our trucks I have found that 4.56 or 4.11 gearing works best with the sm465. This allows you to take off in 2nd and get up to around 15-20mph. Then you can shift into third and as long as you keep it above 15 mph you simply only need to switch between 3rd and 4th during city driving.

I try to run as much gear as I can because more gear=more torque if properly set up.
More torque=more acceleration(if properly set up). This all means that you can use a smaller engine that works less under 55 because instead of the engine working the gearing in the rear is doing all the work.

The 4.11's or 4.56 will increase your highway rpm and you will need to really consider what your engine is happy with running. I don't mind running 4,000 down the highway for short periods because 90% of my driving is under 60mph. The percentage of increased torque in a 4.56 over say 3.73's is 18.2%. That means if your engine puts out 300lbs in direct with 3.73's it will put out 355lbs with 4.56 and that is a difference you can feel.
While 2.73's are great for highway cruising they are horrible for acceleration and require the engine to work harder to move the truck around resulting in worse city mileage. A early 70's Cadillac is a example I use a lot here. They had 500lbs of torque and weighed a bit over 5,000lbs(close to our trucks). MPG wise the car got mileage in the high teens at highway speeds. With all that torque could still let you know you had a big block at highway speeds. But, because of the 2.28 gearing the car had a horrible STOCK acceleration of 15+ seconds. Most are around 17 second quarters in stock trim. Put more gear in the same car (and make the engine oil properly at higher rpm) and you have a tire smoking beast that any Chris Ledoux fan can relate with.

If you find yourself cruising down the highway frequently a T-5 is a great transmission for torque under 350ft/lbs. Yes, I know they can handle more with modification. But, I am talking about one you will pull out of a typical junk yard truck or entire donor with no modification done to it and 100k on the dial.

It will bolt right up and you can use the donor/parts truck parts to make the swap tons easier. The difference is that you get overdrive. I would find a donor truck for less then $600 with a new(ish) clutch/flywheel and a blown engine kind of deal if I ever did a t-5 swap because it will let you have all the little things within walking distance.

For a good mix of acceleration and mileage 4.11's with a t5 and a engine putting out around 330ft/lbs is a proven setup that will get you mileage in the mid teens while still being able to light the tires up from time to time. Most factory trucks came with 3.73's which is a 10% drop in torque and not really worth the 400 or so rpm your going to loose at highway speed unless you spend a lot of time on the highway. I would not spend the cash to do a gear swap from 3.73's to 4.11's though. If you have the donor truck with 3.73 and yours is already 4.11 you have "options".

Now, to be fair don't throw out the idea of a automatic just yet. They can handle lots and lots of torque. You can install slap shifters for that human interaction or just sit back and go. As far as automatics go I am a fan of the th400. They are cheap and plentiful and can handle 500+ ftl bs without concern with little or no modification(do your homework). They are the 350 of transmissions and what you can do with them is endless.
I am actually a bit fascinated with the newer "self contained" zf 9 speeds out now. With a .48 (final) overdrive that means you can run some serious rear axle gear. Soon as someone makes a true standalone package this transmission will become a lightweight hot rodders wet dream and knock down quarter mile times significantly.

I tend to drive stupid and as fast as I can so having gearing that keeps me under 60 in a truck that feels scary going 95 is my method of self control while being able to pull away from most people at a stop light. Your experiences will vary.

Our trucks were not really designed to be fuel sippers. I have found the best way to get mileage in the teens with 4.56 gearing is to put the truck in neutral and coast most of the time. I can get easily double the mileage in my truck that weighs 7,880 lbs (Right now. 6060 empty) taking back roads at 60mph then dropping it in neutral coasting to 50 dropping it back in 4th and getting back up to 60 again then going down the highway at 4,000 rpm and getting 7mpg.
I have clocked 16mpg on many 200 mile round trips using this method. The back roads follow the highways in Mid Michigan with few stops if you use the proper routes and it works because of that. I could never do this in say Pennsylvania and imagine you are going to have a situation with not being able to do this due to the layout of your roads.