The short answer is no. When You enter the a portal from the nether, on your way to the overworld, Minecraft calculates the primary portal coordinates with this generalized equation:
{X, Y, Z} → {floor(X) × 8, Y, floor(Z) × 8}
The game then checks for an active portal in a 128 block radius around that location. Given your nether portal coordinates:
X: -70; Y: 72; Z: -139
Your overworld portal must be within the following horizontal bounds:
X = -688, -432
Z = -1240, -984
Remember, any overworld portals within 1024 blocks of each other will link to the same nether portal, because 1024 blocks in the overworld = 128 blocks in the Nether, and the game checks for portals in a radius of 128 blocks.
If you build a new nether portal at the blaze farm and destroy your old portal, your main base portal will probably link up to your blaze farm. However, when you attempt to go back to the overworld, Minecraft will look for a portal within the above bounds and create a new one if it doesn't find one.
Your best option is probably to build a rail line from your current portal to the blaze farm. Ghasts can destroy any block with a blast resistance below 20.17, but they won't shoot at you without a line of sight, so you can make an inexpensive safety-tunnel around your rail line with pure Netherrack.
You can also build a more scenic tunnel with stone, glass, leaves, fences, etc, since Ghasts cannot "see" through transparent blocks.
This is how it works:
Every time you enter a portal, the game looks for an exit portal inside +/-128 blocks square (y is irrelevant). If you enter a portal at x=100, y=60, z=200 in the overworld, this corresponds to x=12, y=60, z=25 in the Nether. The game scans a square from (x=-116, z=103) to (x=140, z=153), for all y values from 0 to 128. The closest portal in that space is where you appear. If there is no portal in that area, one will be created in a suitable place. Since there is a chance the x=12, y=60, z=25 will be obstructed, the game will search for an open space in that +/-128 blocks square. If such space is found nearby, all will be good, the new portal will lead back to the same one in the overworld.
Note that the game will only scan within the map height. That means that if you place a portal above the nether ceiling, it won't be found, and the game will create a new one for you.
However, sometimes the game will put the Nether portal far from the starting point, if it can't find other suitable place. Let's say it puts the Nether portal at x=80, y=60, z=110 (this is still in the +/-128 bounding box). When you enter that portal the game will search the corresponding space in the overworld: starting from x=640, z=880, it will search the +/-128 blocks - from x=512, z=752 to x=768, z=1008. As you may notice, the original overworld portal is well outside this box. So the game will create a new portal in the overworld. This is what is happening in your world.
To fix that, write down the coordinates of the portal in the overworld (use F3 to get them) and divide them by 8. Enter the Nether, go to the calculated coordinates and create a portal from the Nether. You don't have to be exact, as long as you are within 16 blocks from the calculated coordinates (you need to match only x and z, y is irrelevant). Then the game should find the original overworld portal. Or: Move the overworld portal using the above logic.
In described situation, there is no way to make the two portals lead to each other without moving either of them. I'd move the nether portal and if the new position is outside of the fortress, I'd build a short safe walkway to the fortress.
Here is a crude drawing of the process: 
Best Answer
You aren't moving far enough away.
When you create a nether portal, the game searches in a radius of 128 blocks in the dimension you are traveling to, for an active portal. Since 1 block in the nether corresponds to 8 blocks in the overworld, you will actually need to travel at least 1024 blocks from your original portal in the overworld in order for the game to create a new portal.
Once you have done this successfully without falling into lava, you can make your way to the original portal and destroy it. Then you can create your portal in the nether wherever is most convenient for you, nearby the original one, and your first overworld portals should connect to it when you light it up.
It is surprising that you have nowhere to stand on your portal - I was under the impression that if a portal was created in the air, there would be an extra obsidian platform around it so that you didn't immediately fall off. In fact, if you just don't move when you first enter the portal, it shouldn't make you fall, you should be standing inside the portal. You can then hold shift to move slowly and avoid falling off the edge (holding shift prevents you falling off a block) and build away from the portal till you reach solid ground. If you do in fact fall immediately upon entering the nether, that's a glitch that I've never heard of.
Note: It is still possible to have multiple portals in a small space that all work properly and connect to the right portal. The key is that the game looks for the closest portal in the destination dimension, so if you build both portals in the "right" places (overworld x and z divided by 8 in the nether), before lighting either of them (Obviously you need an original set of portals to do this), then you can connect them properly.