Why do you say it's terrible? A stock 250 makes 150 gross HP, and these transmissions were built to live behind 180 NET hp buick GN and Olds H/O engines. It'll be more than strong enough. They're just as strong as the 700R4, and on top of that, you get a steeper OD gear (.67 vs .70). If you want to swap out the engine for something more powerful, you can always build them up. There are guys with buick GN's running 9's in the 1/4 mile on the same transmission for 2-3 years at a time.
If you want to do this swap, first make sure the bellhousings will bolt up. I believe that all 200R4's were built with the BOPC-Chevy bellhousing patterns, but I"m not sure, and I know there are transmission companies that make adapters for oddball applications. They are the same length as a TH350, so your crossmember will not need modified or moved. However, this is a lockup torque converter style transmission, you will need to devise a way to hook up the tranny. This means that you will have to either get a kit from B&M or something that will lock up the converter for you, or rig up a switch on the dash where you manually lock and unlock the converter.
IMO, it's a good swap, and if you find a healthy JY transmission, it'll live for years. However, a couple things to watch out for - if the computer in the car has been disconnected, or the torque converter has been unable to lock up, it may have overheated the transmission and/or converter, leading to premature failure. Also, the reverse gear has been known to go out on these trannies in big cars, so limit your search to the granny versions of cutlasses, monte carlos, etc. By granny versions, I mean the 2 door, V6 powered cars with no power and ones that people haven't beat on. The big cars were usually used to tow stuff with, and the fancier cars were hot rodded and beat on, so get a nice sedate grannymobile to get the transmission out of.
I will tell you though, that it is nice to have that OD and 2.73 gears. 80 mph is 1800 RPM's locked up and in OD. Cool as heck. Watch out though, because it'll lock up the tranny at 55 mph and you'll be stuck with an engine lugging at 800 RPM's and you won't be able to get anywhere like that.
