I've been working on the trans ID page on the wiki, I just posted a pic of the TH2004R, some call it TH200R4. The other posters are correct..., your is a THM200, and common in G-Body and F-Body with V6 and chevy V8 267/305 and Sedans you will find them all through the platforms and yes some of them don't make sense at all. GM was under the gun to get their average fleet MPG down to make Uncle Sam happy as well as keeping sales and looking for marketing points. The V6 2.8 and the Iron Duke 2.5 got the Narrow bell housing 700R4, internal components the same and can use them for parts, but they won't bolt up to the SBC or the BOP V8's. Most of the late 70's to early 80's with "OD" on the dash and an Olds 307 under the hood will have the THM2004R, yet I have pulled some TH250C and TH700R4s, but not often.
THM200 has eleven bolts on the pan, some stamped "Metric" yet some were not. Was never planned to be used for 4 x 4 so was never designed to have a removable tail shaft. If it has the 4 plug connector on the driver side of the case it is lock up converter. Another sure way to tell is if the input shaft has a small diameter stub on it with an O-ring on the end, its a lock up, as the O-ring is needed for secure fluid coupling to operate the lock-up.
The idea behind the THM200 was lighter overall mass and less rotational mass, and this was supposed to equate to higher fuel mileage. Racers are now finding benefit of the lower rotational mass, but the internals are mostly poor quality cast or thin stampings. High speeds (RPM) and any heavy torque as the other poster mention, tends to turn the inside of it to paste. The nice thing of the THM200 is very low first gear ratio and a wide ratio between 1 to 2 shift.
Issues of the stock unit is a lower number of friction and steels, light weight stampings, and weaker sprags. Sonnax and Mako Trans Parts have some of the upgrades.
THM200 or the THM200C is a good cruiser, definitely needs a deeper pan and external cooler, but is reliable if maintained properly and if you don’t mash on the gas as a regular thing. Summit has the lock up kit for about 75 USD, Sonnax or Mako has the Servo unit that is highly recommended.
I getting ready to build one this month so can post some pics, overall its not a bad basic trans. If you do not use it set it to the side, sometime when I’m in the area I’ll buy it up from you.
The 700R4 is excellent Trans, basically a THM350C with overdrive and can be built to some very impressive levels. Just remember if swapped for a 3 Speed it has an extra detent so the linkage needs to be adjustable to accommodate this. It needs to be properly matched to the rear gears. If you don’t use the OEM ECM you need the after market lockup kit. If the car is going to be custom build, then from the beginning decide which one to use. Swapping from 3 speed to 4 speed (3 + 1OD), you need to design in the linkage or cable driven shifter such as the B&M, rear gears, drive shaft length, etc… . B&M has both universal shifters in both 3 or 4 speed.