I did this fan conversion and am very pleased with it.
I bought mine on ebay, here's an example:
http://http://cgi.ebay.com/ebaymotors/Lincoln-Mark-VIII-Electric-Fan-/270667541337?pt=Motors_Car_Truck_Parts_Accessories &hash=item3f050aeb59
I also second the recommendation of using the Hollister Road relay setup.
http://www.thehollisterroadcompany.com/default.html
I am running twin pusher fans as well, but only for looks/authenticity purposes. The Taurus/Mark VIII unit moves plenty of air. No more overheating problems since I installed the unit.
FYI: I have the low speed triggered by my thermo switch, then installed a 3 position Lucas toggle operating the twin pushers (position 2) and the high speed puller (position 3). If you go this route, be sure you
don't use an
off/on 1/on 1&2 type toggle. You need the
off/on 1/on 2 type 3-position toggle. Don't ask how I know!
Matt