anything beyond stock requires floor surgery, how much and how extensively you need to mod the trans mount depends on the power transfer unit of your choosing. (the easy button here is a gearvendors unit)
I'm curious how the Rallye dash fits vs what is in there, as the Rallye appears to be a much longer, once piece, whereas what is in there now appears, at first glance, to be a piece that just contains the gauge cluster and doesn't overlap with the radio/ac control region.
the rally dash is its own beast. the frame itself is different from the standard dash frame (and the wiring). @72bluNblu modded the dash in his duster, so have a look at his build thread for the info there-- and inspiration! it's a great build.
rally clusters show up here frequently, or you can buy new or "versions" of it from classic dash, thunder road, classic industries. i *think* dakota digital was working on or has a johnny hot **** new complete unit, but i can't find the link anywhere.
re: your question on sourcing parts. the classifieds here are a great resource. then you have the usual: laysons, year one, classic industries, DMT, mancini, jim's, etc. swapmeets are where you'll find the big chonky parts.