Used Transmission

[one9six9]

I need a transmission. I'm trying to get by as inexpensive as possible since I don't really know how long I will have a three speed set up, but I don't have the money free to upgrade to the 700r yet.
I have a turbo350 with my 250 I6. I found someone that has a turbo400 that was pulled out of a 69 Chevy Chevelle two years ago. He says that it had about 5000 miles on it when it was pulled. It also has the stock torque converter. I don't know what size engine was in the car, but he thinks that I will have to get a new torque converter because the flywheels are not the same. If this is correct and the flywheels are not the same can I just swap flywheels, or are the bolt holes different from one torque converter to another?
Oh yea, is there anything that I need to look out for on a used transmission, or is there anything that can be checked being that the transmission isn't installed in a car?

Thanks,
Keith