New guy, What tranny should go behind a 360?

Well spends on what you're gonna do with it. 727 is a good tranny, however if build up a 904 you'll get more power to the ground because it take less HP to power it. If you want an overdrive tranny you can either add a gearvendors unit to either the 727 or 904. Something else you can look at (if you're not a stickler about it being all Mopar) is the GM 200R4. The 200R4 is a overdrive tranny that is smaller then both the 904 and 727 (this way you won't have to mod the tranny tunnel) and all you need is an adaptor and a 12-volt source to get the tranny to go into over drive. The 200R4 is a lot like the 904, doesn't take much to power it and once built/beefed up can take a lot HP and TQ.