I just bought and received a used 4x4 extension housing for my 4R75W from Whatever it Takes Transmission (found it online). Only $33 plus reasonable shipping and it looks like it's in great shape. They have a few more too.
I'm not sure what tranny you have but here is the link to the extension housing, hope it helps and keeps your costs down on the build:
http://www.wittrans.com/c-724-Ford.aspx?pagenum=38
Part number: RF-F2TP-7A040-BA
U76770B Extension Housing, AOD/AODE 4X4 (6 3/8" Long)(Aluminum) (With 2 Threaded Bosses For Shifter)(Cast # RF-F67P-7A040-AA) $33.38
U76770BA Extension Housing, AOD/AODE 4X4 (6 3/8" Long)(Aluminum) (With 1 Threaded Boss For Shifter) $33.03
U76770BC Extension Housing, AODE/4R70W 4X4 (6 3/8" Long)(Aluminum) (With 2 Threaded Bosses Threaded Bosses For Shifter)(Cast # RF-F2TP-7A040-BA Or RF-3L3P-7A040-AB) $33.38
Also, found the 4x4 output shaft on eBay for $55 and it looks almost new.