Guidance on selecting auto trans for 600HP 69 AMX

I have a TH400 in my 1968 AMX, with no modifications to the transmission tunnel. You will either need to make a custom cross member or modify the original to fit. The biggest downside of the AMC pattern TH400 is the lack of speedometer hole, and the annoying 5deg twist built into the bell housing.
The AMC pattern TH400 was also never made in a 2wd version. That means a full transmission disassembly to swap the output shaft to a 2wd version.
This transmission requires a special TH400 flexplate. The stock one off a Jeep is too flimsy for 600 hp.
JW makes an aftermarket bellhousing for the TH350 and TH400 to modify a more common bop or Chevy pattern case to fit an AMC. They might be able to make one for a 700R4 if you want overdrive.

I have read reports of the 727 fitting in the transmission tunnel as well, they were supposedly able to use the stock cross member with a transmission mount from a 1972 Javelin.
SFI rated flexplates are easier to come by for this application.

Both of these transmission options require a crankshaft with the later 72-up crank flange. While the TH400 is slightly more durable, the TF 727 is probably a much better fit and easier to come by.

Thanks guys for the quick responses.  As I understand it, the original auto trannys that came in the 68-71 390/401's were the Borg Warner M12, and that AMC changed to the TF-727 in 1972.  Should I assume that finding/rebuilding an M12 would not be an option?  

zim390 wrote: Thanks guys for the quick responses.  As I understand it, the original auto trannys that came in the 68-71 390/401's were the Borg Warner M12, and that AMC changed to the TF-727 in 1972.  Should I assume that finding/rebuilding an M12 would not be an option?   So if I go with a TF-727, this is my understanding at a high level of what I would need to do: 1. Machine my 390 crank to fit the input shaft of the 727.  2.  Rebalance engine with the new 727 flexplate.  3.  The driveshaft must also be cut and rebalanced. 4.  All mechanical 727 kickdown linkage must be obtained, installed and adjusted. 5.  Install new shifting rod linkage. Does it sound like I'm in the ballpark? I also see a bunch of 727 tranny suppliers on the Web.  Are you aware of any that may specialize in AMC's and may have already supplied to AMC owners?

Machine my 390 crank to fit the input shaft of the 727. Machine work is for enlarging the torque converter pilot hole only

I have a 727 in my car. But I like the 400 better for extreme hi performance. But either will do the job. A 360 TF can be built to handle 600 HP with a bolt in sprag and shift kit.

zim390 wrote: Thanks Harry......lots of good info.  You mentioned the 998.  Perhaps I should also be considering that or even a 904 as well?  My head is already starting to spin, as there seems to be numerous options. I am taking the time to search the entire forum for additional info, as I assume others before have had the same questions.

Yes I have a built 998 in my Spirit behind a 400HP 360. The 998/904 are basically the same except the 998 was created by Chrysler to handle the HP from a smaller V8. AMC used them with 304s from 72-75 or so in non lock up and then in 304s and Eagles (999) till the end in lockup form. It has a couple reinforcing ribs externally over the rear servo to help identify it.Internally it came with a bigger front drum and a double wrap band. I like the 904/998 in a light car under 500 HP. The small block Mopar guys love them and some Pro Stock guys run them in Big Blocks for racing as it uses less HP than a 727. There is even a high dollar racing trans out there called a ProFlite that uses 904 guts in a 727 case but I like the durability of the 727 in a bigger car on the street. Also converter choices are limited for AMC 904s