it's time for you to figure out how to weld and fabricate stuff. or plan on dropping a minimum of $10k at a shop to do that work. it's a lot of work, not hard, just a lot of hours.
a mustII transplant requires:
new master cylinder, which requires: either a firewall mount or a frame mounted master cylinder. either one requires welding and fabrication or buy an expensive kit.
also you will need all new brake lines, again welding brackets.
since you need to remove the old front x-member you will need to fabricate new motor mounts. a little scrap steel and some welding and grinding. no kit for motor mounts.
a new steering column or modify the old column. welding brackets again.
a rearend upgrade requires:
welding a c-notch above the axle and boxing the frame.
cleaning up the new rearend (grinding off brackets) and welding on new spring pads.
rebuilding the springs and shackles, disassemble, clean up and painting the old springs.
a full blown cut out springs and weld fab a new 3 link, 4 link, air bag or what ever...
it's a lot easier to do all this work with the sheet metal off, and since the sheet metal is off you might as well replace the rubber mounts under the cab.
all the above is a lot of hours, but nothing too difficult to handle. my truck? $1900 for a hub to hub mustII, $200 gen II rearend, $35 astro minivan steering column, $200 03 must hydroboost and MC, $100 spring shackles, $30 axle flip parts, $250 ss brake line kit, and a lot of little parts and trips to napa. add it all up and i might have $3500 in parts and a lot of time (i don't want to know how much time either). but no where near the $10000 that a shop would charge.
take the extra $6500, buy a welder and learn how to use it.