I'm going through the same process on a 68 coupe...
Don't bother with a "donor car", it is more work than it is worth... also check the floorboards while you are at it...
I went through
http://www.cjponyparts.com/ . I've had pretty good luck with them, and was even able to save a couple bucks on some of the parts that I needed through their ebay auctions. Mine were shot... and since I'm not one for tearing things apart after I put them together, I decided to redo most of the front suspension while I was at it...
Costs were:
$115 for set of full floor pans. (via Ebay)
$350 for a starter suspension kit
$50 for a new idler arm.
$90 for a set of torque boxes.
$250 for a set of full frame rails...
A ton of time... a 10lb spool of mig wire and gas to go with it... a 4 1/2" angle grinder with some cutting wheels and a wire brush attachment... primer, paint, seam sealant... a garage... and a really big broom.
Also search around for measurements... I was lucky enough to find the measurements for my 68 on this website, not sure about the '73. I also TOTALLY cleaned out the engine bay of damn near everything... engine... tranny... washer container... and so forth... as well as the interior as I needed to pull out both floor pans...
Best of luck... it is more of a job than I thought it would be, both in time and money, (luckilly I'm a welder by trade, so that part of the equation isn't too much of a hassle for me) hopefully the end result will be worth it.