Chrysler 904, 998 & 727 interchange information

I've got a Gremlin that was formerly a six, now V8. Temporarily I've bolted the stock 904 to the engine just to try it out and make sure things work and are adjusted. Differential is already swapped but I do still need some work to reinforce the rear leaf. My last and hopefully final change will be the transmission. For now I'm running a vacuum secondary carb and stock converter to prevent early damage to the tranny. Not going to be a drag car. This is a purely for street use and maybe some hot rodding as the opportunity arises.

My question is as follows:
What can be swapped in as far as transmissions go with the least work? I want to avoid having to fabricate a drive shaft if possible. I've been told the 998 transmission parts will swap out into the 904 case I already have. Also, how different is the 727 transmission and what would I need to make a proper swap out?

Thanks in advance

AMC had an unique bolt pattern to the engine. They are all good transmissions and the 904 and 998 can be beefed up fairly easily to more than handle a mild V8.

AMC patterns are unique to AMC, so a mopar will not bolt up. However the internals will work.

I would agree with what ken said. The problem with a 727 in a Gremlin is it is a very tight fit, almost to the point of being unworkable. Both transmissions have good and bad. The worst of a 727 is the low sprag, it is just pressed into the case and held by serrations and a small set screw. Try the netural drop thing and the case is junk. It can be fixed with a aftermarket bolt in sprag but they are expensive. The sprag is rivited to the case on 904- 998 transmissions. The clutches in a 904 are way smaller than the 727 ones but more can be added using the right parts. Use a late pump with the flats to drive the converter. Unless you are running huge horsepower in a heavy car I see no reason to use a 727. 

Most everything I have done has been done on a fairly stringent budget so I am answering from that stand point. That of what can I expect for the biggest bang from my buck and enjoy.
I believe what ever you choice is you are going to have to spend some money.  That said, and assuming a Gremlin is not a heavy car it does not make much difference at all what transmission you choose, the car itself is not forcing  you to go heavy duty in terms of maximum beefy.
A 727 is an S.O.B. To fit and expensive to rebuild.
But it is almost bullet proof.
A 904 is also expensive to upgrade to something that will take more abuse, but it is not only do-able but has been done over and over again and it fits as does a 998 which if you find one from behind a 998 is a better choice but harder to find.
That said my preference is to take a later 904 with a lock up torque converter.  Rebuild it to handle more abuse, it is when done adequate and more efficient than a non lock up 904 and any 727.
A lock torque converter than will act as a poor mans 4th gear in that when it comes up to speed it lowers the engine rpm by about 500 or so rpm.
That is done by changing spring tension in the valve that controls the lock up point so that it will lock up at a higher road speed,  say 50 or 55 mph rather than 39 mph as designed.
Who does that?  An old school transmission shop may have some one in it that is familiar with where to get the parts and can handle that as part of the up grades.
That was a common modification back in the day.
I then would have a 2200 rpm Stall lock up torque converter built which will make the car much more drivable and be more flexible in how it can be driven.
As to reliability?  If you take some care of it, it should be more than reliable.
I am running that package behind a very healthy (about 180 hp)  in line 6 and have close to 50 or so thousand miles on it with no problems.
Or for a little extra money, put a 200r or a 700R4 in the thing and get a modern 4 speed transmission and have a 2200 rpm stall converter built for it with the lock up feature.  It don't fit either, but you would not regret the effort to make it fit.
I am doing that in my Hornet.
A drive shaft can be made up for about $150.00 and a 2200 rpm stall converter is in the same price range.
And an appropriate electronic lock up kit is $100.00 +/- some.
Lokar makes parts you can use to shift it with the column shift off of your Gremlin.  That is what I have done with my Hornet.  The lokar parts run about another $100.00 or so.

I would go a 700R4 or it's sibling before I spent money on a 727.  The cost has to be close the end result would be more satisfying.
Not wanting to do that I would try to find a buildable 998 and if I couldn't I'd build a 904 and call it good to go.

In fact?
That is what I did.