There are according to the information I have on hand, 13 different types / style of 727 transmissions. The year and the engine size are very important.
From 1978 and on, you have a lockup style transmission, and a non lockup style transmission to contend with also. The 727 was also used by American Motors and International trucks.
If the transmission you are looking at has a large brake drum on the extension housing, that transmission came out of motor home, and the brake drum is the parking brake. There are 8 different extension housings you also have to take into consideration.
For your Challenger, as Dave pointed out, the overall length is a major consideration. If you can find the transmission (out of a 1971 to 1977) that is behind the same engine as you are using, then the only items you may have to change would be the extension housing and matching output shaft for your particular application.
Hope everyone had a great summer, 'cause I sure did.
transman