Get a rebuilt inline 6 that came from that car model. Anything else will be a bastardized combination that doesn't belong, won't fit, be hard to make work well, probably not handle as well as the original, cost a ton of money, probably won't be as reliable, look funny, and will bring down the value of the car.
Unless you're looking for a project that will take long hours and probably not return much in the way of a driveable car for a long time, stick to the motors that grew in that engine well. Swapping in anything else will be a lot of work. And it will be about 6 times harder if you don't have a well equipped garage to do this in.
