I also modeled a 10m-double extended Zep fed with 450 Ohm window line, for multi band use 20m-10m. Modeling the doublet and getting the Z's is OK but I don't seem to find good feed line length with average impedances to tune easily.
See pages 4-5 of the following:http://www.dxengineering.com/pdf/WA-Multi-Band%20DipolesRev2.pdf
will I be able to connect the vertical's radiator and the elevated radials to it &how should I do that - the MFJ has coaxial connector?
I guess you would connect the SO-239's center pin to the vertical's radiator and the SO-239's shield to the radials.
how can I measure at the input of the 450 Ohm parallel feed line int the shack for the doublet antenna?
"Use the ANTENNA connector’s shield for one lead and its center pin for the other."http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
I hope this helps. I recently acquired one of these but I haven't tried what you want to do yet.