During the time frame of FMJ's, they made 3 different styles of the A904 for two different engine classes.
No A904's (from factory) were ever made to fit behind a big block.
The A904 engine groups are for /6 and for the small block.
I use the term “A904" – just because I've used it most of my life, but the different versions of that transmission family are:
A904 (-'83?) - Light Duty
A998 ('79-83?) - Medium Duty
A999 ('76-'89+) - Heavy Duty (HP, Police or Towing)
Of those three, there is the lockup and non-lockup (converter) versions.
Note: I have conflicting data about when the A904 and A998 were no longer available for M-bodies.
Most of the A998 and A999 (but not all) have the more desirable low gear ratio in it (which is what many racers use or will modify to make fit into the above mentioned big block).
Lockup transmissions came out mid-year 1978 – but, was an option to “order” non-lockup for many years afterwards. The key word is car was “ordered” that way.
All of the cars on dealer lots all came with lockup transmissions after a build date of early 1978.
The wide ratio gear set (the desirable one) came out in 1980 and was fully integrated into the A904 family by 1981 – so all '81 and newer A904 appearing transmissions have the good gear set in them. None of the '79 or older transmissions have it. Some of the '80s have it - but not all.
The best way to tell – as well as to make sure of any other replacement parts is to get the numbers which are stamped on the oil pan rail on drivers side (under the shifter levers) which is called the TIN (Transmission ID number) – which ties into
your question # 2
View attachment 40160
View attachment 40161
The last 8 or 9 digits of the VIN are stamped onto a pad on
other side of transmission – which doesn't help us in this aspect.
The A904-A999 can be a good transmission choice. It is smaller, lighter and can handle a lot of HP in a modified form.
The A904/A998 – not as much HP in stock form
All A904/A998/A999 all were the same length and
all used the small slip yoke (aka 904 slip yoke – 26 spline / 1¼” inner diameter)
All (car) A727's were the same length - but longer than the A904 family (roughly 4.1" longer). Note: they also have a 4*4 version and a HD truck short shaft A727 – but neither are suited for car usage (no slip yoke).
The A727 slip yoke is 1-7/16” inner diameter / 30 spline.
I'm going to intentionally not answer your
question #3.
BudW