My 1.8 plan is to allow players to select limited above ground areas that they would like to keep and then move those by hand to a fresh 1.8 map using mcedit. In addition I'll set up a portal between worlds to allow them to move their other items like rails, torches, chests, etc by hand themselves if they want to.
I don't know of any "automated" stitching tools. Most people I've heard either do what I'm going to do, or just start with a fresh map and let their users move their own buildings.
edit
There is now a stiching tool.
http://www.minecraftforum.net/topic/629884-a-tool-for-merging-171819-maps-mcmerge-v04/
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.
Best Answer
You can try using structure blocks, they will save the structure as an NBT file, which you can put into other words.
Limits
Java is limited to a 48x48x48 space, so this might not be too practical as so many structures would be required to do this, assuming you have a fairly large area you want to copy.