Okay, so this isn't going to be something I buy right away, but basically my plans for my '89 RS are to build a 383 Stroker, and the setup I'm using is going to put me around 550HP/490TQ, according to the CamQuest program that comp cams has. After I build the engine, I'm going to buy a brand new transmission, Automatic.. and I'm kind of wondering what I should go with. Currently have a TH350, but also looking at 700R4, and I've heard you could use a TH400 with a shortened driveshaft or something like that.. I'm also gonna need a new torque converter and a shift kit for the transmission. Probably spending around $1,500(?) on this part of the project. Any recommendations as to what brand/type of tranny, and which torque converter I should go with? Thanks.

Call Dana @ Pro-Built Automatics. Easy to work with and many people on this site use his trannys.http://www.700r4l60e.com/
He is currently building a 700r4 for my 383 build.

Definitly, if I can get a transmission built specifically for my engine that would be great. Any other takes on this?

Well it breaks down like this:

700r4:

pros:
OVERDRIVE!

Cons:
Complicated
expensive
cant be built tough up but pricey
must start with a good core (earlier versions have issues)
difference between gear ratios 1st and 2nd to big

TH350:

Pros
Simple
reliable
easy to build
cheap
lighter than the th400
easy install

cons
weaker than the th400
no overdrive

TH400:

pros
Bullet proof! Even in factory for but can be built for extreme motors
reliable tried and true design
easy to build
cheap

cons
little on the heavy side
no overdrive
Not exactly a direct swap


Personally i would do a th400 if you honestly planning on putting out those kinds of numbers especially if you'll be running slicks as a built TH400 > built 700r4. If you've got some real money to throw at this the 700r4 would be nice as that overdrive is real nice to have. Even then though under hash driving conditions i dont think it would be reliable as the TH400. The only one i wouldn't use is the th350. Not that its a bad transmission by any mean it just doesn't do anything particularly well. It has no overdrive and its weaker than the th400 so there's no real reason to use it other than the fact you already have it.