As stated above, find the 1973 Service Manual and Body Manual, that way you have reference how it all fits together. Also, as stated above, need to shop all sources for prices and availability. Don't trust that just because a part is listed on a webpage means that it is in stock. Call and verify to avoid the dreaded "backordered".
I have used Classic Industries and have not had any issues. They may be higher on some things, but then they may be the only one that has a particular part, too, so if you want it... Same with Year One. Haven't had any issues, prices can be high.
There are many Mopar specific vendors, like Mancini, Tony's Parts, Monicatti, and more in addition to Year One and Classic. Check eBay and Craigslist for parts, too.
Check Rock Auto, Summit, local parts stores for speedo cable and fuel sender. May be able to get new switches locally, or check eBay for rebuilt ones.
Good luck, have fun.