You can use a stock crossmember if the transmission mount attaches at the tailshaft like in Persh's picture.
http://www.persh.org/images/65Pickup/Trans_Xmbr4.jpg
What I ran into with my 700R conversion was the transmission mount was further forward on the transmission.
This location interfered with the pan, so I trimmed the front of the crossmember—mainly because my truck had the parking brake assembly attached to the rear of the crossmember and I didn't want to lose that provision.
If your transmission mounts at the rear, it is true you can use a stock crossmember, and depending on your parking brake setup will have to either attach it to the crossmember or use the later style like Captain Fab suggested.
If you want the crossmember with the parking brake attachment already done, Bowtie Overdrives has one for $175.
I decided I didn't like the modified crossmember in mine so I went with the $79 CPP one. And while I'm doing it, using the later style parking brake setup.
Parking brake assembly:
I have a '70 c20 longbed parts truck in my yard. I don't know if the parts I removed are available aftermarket or you would need to go to a junkyard, but I can say with my install, all I need to buy is the intermediate cable. LMC carries it for $9.95, and it is the same part number '64-72, so even though the parking brake is different (hand brake vs pedal) the intermediate cable is the same.
I agree the mechanism is simpler, but I don't know if I would have gone to the trouble of converting to the later style if I didn't have the parts truck sitting 50 feet from my garage.