In case you didn't already know if you are going to do this make sure you taper your frame splices so you don't create any stress risers in the frame when welding. The picture that McBride posted above shows this. You want the ends of your added section that go over the existing frame to be like <> not like this . Do a quick google search on chassis fish plates to get some rules of thumb for spicing frame sections together.
Thanks for the tip.
Since I am working with the short wheelbase I can only overlap the frame by 8.5" or so (working with 17" of straight section between the trailing arm mounts). For that reason i can't fabricate a conventional fish plate ends and sacrifice the overlap length which is more important. Instead I'm going to tie the extensions into the trailing arm mounts and the cross tube where the original frame is reinforced by them.
I've actually just fabricated the outer extension angles and cut the frame. I'm posting more details in my build thread viewtopic.php?f=18&t=16525
Once I finalize the shape of inner and outer extenders I will post a dimensioned drawing in this thread for those who are interested