overdrive four speed C10

[iffray]

i'm picking up a 1969 C10 Lbx this weekend, straight six... three on the tree..
long story short, i want a four speed manual with overdrive.. if it's even out there. five speed is good too i just figure that equals more work & money and harder to find if available at all..
any hints?
i really just need a starting point ie.- what trans will work with what i have.. i'm an apprentice mechanic in a shop that does resto/body/mechanical, so having to convince a trans to mate up is no problem. i do plan to drop an eight in there eventually and don't really want to buy another trans to go with it, so interchangability is great, but not a necessity.
also, if anyone has one to sell, i would surely consider it. i'm in chicago though.


thanks in advance

john