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
I believe, basing on ReallyGoodPie's answer, I do know something that would work most of the time.
Unless you're a devout traveler and wandered for days away from the spawn point (not too smart, a creeper blows up you and your bed and you're back to spawn, days away from home - not to mention all three strongholds are between 640 and 1152 blocks from the spawn) we can assume your Nether Portal was somewhere within, say, 3200 blocks radius from 0,y,0 (that's like 9 maximum zoom-out maps!). Since coordinates in Nether are an eighth of the Overworld, that means the portal will be within 400 blocks from 0,y,0 (and you can estimate the radius by estimating the distance of the overworld portal from spawn.) You can also estimate the altitude (y) - 64 if that was the surface, more for hills, less for underground.
Now locate 0,y,0 using F3 and then travel in spiral around that point, trying to explore increasingly wider circle of terrain and controlling your distance using the coordinates. That way you shouldn't miss any terrain and be able to locate familiar areas and eventually the portal - 25 chunks radius isn't something beyond hope for exploring, and most people will have their homes (and portals) within 1000 radius, that's mere 8 chunks of Nether away from Spawn. (and if you remembered rough direction of your portal from spawn, using the Cobblestone Compass trick you can head straight for your portal from the "origin")
Another option, inspired by Loren: If you have enough wood, or a chest - or are near enough from the portal (say, aforementioned 0,y,0) - or are patient enough to locate a treasure chest at a fortress - write down your coords, dump all your equipment, suicide, then go, retrieve it all - stopping long enough to take note of the portal location this time.