If you want to run Stock, that's a little different. In Stock, you MUST run an engine and transmission (type, not exact transmission) found in the guide for the car.
You could not use the TH400 with the brake, nor the 9" in Stock. Not a big problem. Sell them, and replace them with a legal transmission, and one of the new 12 bolt rear ends from Mark Williams, or maybe Moser.
The problem with Super Stock is that you'll need to take your car to a good chassis builder and have a new 4 link system put in it, the wheel wells stretched, the cage modified to work with the new pieces, and the front crossmember will need to be heavily modified. You could drop $25K easy, plus the cost of modifying the 9", which negates the cost benefit of keeping it.
The two 350 engines, the 255 and the 300, have been flogged really hard, so you have to have a really good one to run around 1 second under, which you really need to do to be competitive in F,G, and H Stock, stick or automatic.
The 302 is also well flogged. And requires a 4 speed.
However, the 4 speed is pretty cool, fun to drive, and low maintenance. It also greatly reduces the odds of getting a heads up run against a car you can't beat. Most any of the combinations for the 69 Camaro can be run with a 4 speed, and you can put complete new Jerico set up in for under $7K. A good friend of mine, Kevin Helms, is a multi time champion (divisional and national) and event winner with his B/S 69 Camaro.
There are actually quite a few big block combinations that work pretty well for the 69 Camaro, especially with the 4 speed. The best one may be an iron head 396/375 4 speed. The draw back to the big block cars is that unless you run a 4 speed, the odds are good you'll run into a new Challenger or new Mustang that you cannot beat in a heads up race.
A nice competitive Stock Eliminator big block can be built for maybe $15K or so. No, it won't set records, and it won't beat a new Challenger or Mustang, but it will be competitive and fun, as well as not terribly expensive.