Add a gear!
...gain MPG and MPH at the same time....
Most (if not all) of the 521-620 pickups had a 4.375 rear end. Compare this to the 3.900 found in the 510's, or the ~3.00 found in American V8 cars and trucks, and you will see a recipe for high-revving freeway driving. This makes the engine work too hard, drink too much gas, and make too much noise. The solution is a five speed!
77-79 200sx five speed swap
This should be the easiest changeover, and the only one that will allow use of a bench seat without some serious wrangling. This tranny is called the "Dogleg", referring to it's goofball shift pattern -

Dogleg 		Standard
R 	2 	4 		1 	3 	5
1 	3 	5 		2 	4 	R

Strange pattern aside, this trans is unique as it is the only "short tail" five speed Nissan made. It has the same length as the stock four speed, so the only needed modification should be a transmission mount. But please note - the 1600's two-piece driveshaft may not work properly. A list member commented that the 1300 driveshaft is one-piece, and works perfectly with the dogleg transmission.
Other five speed swaps
Just about any L, Z, or CA series transmission can be used. Z and CA trannies require some bellhousing changes and modifications to fit an L series engine, while the L transmissions will bolt right on. All of these transmissions will move the shifter back about three inches, and will require shortening of the driveshaft. A preferred method would be to use a one-piece driveshaft, which requires cutting out the carrier bearing. One list member took the complete drivetrain out of an early-eighties 720, and dropped it right in, only custom modification was to the trans mount.

Here's a post from Dave:

    I have gone through about 4 transmissions in my truck so I finally went to 5 speed. I had no help from anyone and I was fairly new to Datsuns so I asked around and I was told you could not get a five speed into a 521. That is a bunch of nonsense. I used a 5 speed from a 620 so I am not exactly sure about the 200sx trans but as far as I know the 200sx trans drops right in and you can use your stock seat. But I heard that the 200sx trans is just as weak as the stock four speed and it has that funny shift pattern.

    Anyway with the more common truck 5 speed, I think it's the better swap but it takes more work but once you've done it you can find replacements very easily at any junk yard. The 521 has a removable trans mount that bolts to the cross member, you need to fabricate a shorter one, what I did was I made a 90 degree saddle that bolts up to the cross member and the cut the original mount down to the proper length (need trans in to measure proper length) then I welded the piece of the old mount to the new saddle, this is the worst part but not too bad. You need to remove the trans cover plate from the inside of the truck, I put the trans in with the engine in however this is a challenge and takes a little forcing. If you do it with your engine in you will need to remove your clutch and pressure plate, you will put the clutch back when the trans is half in otherwise the trans won''t slip in. With no trans mount or shifter wrestle the trans in, hope you're strong. You might bend up the hole in your floor but nothing you cant fix easily. It will go in there now with the trans as far back as you can get it put the clutch back in there is just barely enough room to get it and a clutch alignment tool in you have to turn the motor over by hand to get to all the bolts, I can turn my rebuilt 1600 over by hand fairly easily just watch out not mash your fingers that ring gear is sharp. After the clutch is on slip the trans into place and bolt it up.

    Now measure for your mount and make it. You will also need your driveshaft shortened, I am not positive but I think the 620 5 speed drive shaft might work. You will need a 620 drive shaft or at least a front yoke as the 521 had a bolt on flange at the front of the drive shaft and the 5 speed uses the better slip in style. That should take care of the underside except for some wiring just get a manual for the truck you got the trans from and compare to the 521 wiring.

    As for the interior you need to customize the trans cover, I cut the raised area around the shifter hole off and welded in a flat plate that was bent on the edges to match the bends that were already there, then you figure out where you need a hole and use small, maybe 2 inch hole saw to cut a new hole you don't need much of a hole. Then find some shift boot at a parts house. The only other thing is the seat, you can't use a stock bench seat. I have Honda Civic buckets in mine so I have no problem but if you want a bench you can get a newer Datsun seat with the cut out for the shifter. It's not easy but it is worth it. You can find the trans in 620s that are 5 speeds be careful there are four speeds with the newer shifter but they have a plate that bolts on in front of the shifter the five speed does not. The 5 speed 280z also has the trans. The early 720s that have the L20b have the same trans. You can use the 5 speed off a NAPS-Z but you have to put the L-series bell housing on and change 2 bearings to put the bell housing on it.