Hey Dave...
Been there, done that.
When I got my '67, all the A/C had been yanked out from the firewall forward so the PO could have room for his 351C (the swapping with a 302 of which allowed me to get the basketcase, errrr....car....for 1 grand). Anyway, I digress (as is common with me...but I digress yet again...)
I replaced my evap core and heater core with all new ones from Mustangs Unlimited (as I recall). I got the seal kit for the box and the check valve (along with all the engine compartment and mounting stuff and compressor) from John's Classic Cougars. I already had the vacuum can. The water valve you can find on Ebay (or from your local PepBoys/Kragen - which can also provide the heater core, for that matter).
It took some time, but it's all hooked up now, and worked GREAT for about 4 days until the freon leaked out...(after being fully pressure/leak tested by the guy who filled it - *sigh*) So aside from that, it's not an impossible project, and it's great to have it all working as Mr. Mercury (who?) intended.