A999 tranny same as 904?

Been trying to figure out the ins and outs of the 999 transmissions for myself. Apparently the 999 was originally behind vehicles with a 360, the 998 was behind 318 equipped cars. First years for these transmissions was approx. 1974 or so. The signifying feature of these trannies is the 4 and 5 clutch drums. Regular service 904s from the same era generally had 3 clutch drums.

In 1978 some/most of the units got the lockup feature. In the 1980s they pretty much all got the lockup feature. The input shaft and pump on the lockup units are different than the non lockup ones and there's not a lot of interchange without careful choosing of parts.

To my knowledge, the low gear set was introduced in the 80's but it's my understanding that the low gear set could only be found in lock up transmissions. I could be wrong on this though.

Best unit to have is a non lockup one from the mid '70's from a 360 equipped car. Get a low gear set, slap it in there and you're ready to rock. I got one from TFlite Patty on Ebay for around $100.

BTW, the Mopar chassis manual says the 999 is the quickest trans to use.