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
Not only is this unfortunately impossible with your current setup, it's impossible to reach a high overworld portal from the Nether as of 1.2.4:
Additionally, without mods, there is currently no way to move upwards without some sort of connecting structure, save perhaps a vertical TNT cannon, and it seems to me that you want something a bit more usable than that.
Now, in theory, once this bug is fixed, you could build a transport bridge a few blocks out in the Nether and the same number of block multiplied by 8 in the overworld coordinated just so that the higher platform is closer to the new portal, but it would be complex to do.
Until then, you're stuck with regular transportation.