Need help with a trans swap

[Freddyj323]

Alright, my name is Josh and i bought my first 69 chevy c10 pickup about three days ago. It was originally a 307/ 3 speed manual but the guy put in a 327 instead. Its the hugger orange with white top. Only rust is on the drivers side rocker and i already got it fixed. Now the big question, I really want to swap out the 3 speed for an automatic. Im told a TH350 would probably be the easiest but i do have a brand new TH400 sitting out in the garage. Now i am wondering what adjustments i am gonna have to do.... shortening the driveshaft...get a new driveshaft.....will it bolt up to the engine....moving the cross member....something about a pilot bearing??? Help?

Thanks
josh