What is your game plan? Will a budget control your build before you tire of this car? What is your emissions situation?
The 200-4R happens to have a bolt pattern for both families of engine blocks and will live behind a Chevy 350, depending upon how tired it is. You will have to figure out the torque converter lockup strategy. It has overdrive, but a similar first gear as a TH350, which means waking up your car might involve a gear change to 3.08-3.73 gears (and install a limited slip at the same time). If you want to race, be careful with the stock 7.5 rear.
If you dont plan on more than 300-350 hp, the 700-R4 may offer you an option to keep the rear gears you have, as it has a steep first gear. People rag on these, but for modest builds, they work just fine, especially if has already been rebuilt. You can replace the pinion gears within the differential with a Powertrax unit you can install yourself in the driveway.
The 700-R4 will also need a torque converter lockup solution. Painless, B&M, and others make a pre-computer kit to use transmission pressure and engine vacuum to engage the lockup. Check my profile for a link.
If you decide to ditch the 200-4R, let me know. I need another one for my Olds. If you currently have a 307, before you toss your computer/ECU, let me know; the same Olds needs a PROM change to go from 3-speed to 4-speed whether you sell me yours or not.