Your original driveline combo should not be unreliable. It's just a matter of having the work done properly.
I can fully understand how an unreliable vehicle can be frustrating, especially if you have already had lots of work done to it. There should be sources out there who can do the work right. The original Caddy driveline is very straightforward and easy to repair.
A modern driveline will be expensive to buy, expensive to install and will decrease the resale value of your car. However, if you still want reliability, go for a small block Chevy engine, with a carburetor and a THM 700R4 transmission (non computer controlled.
And keep all of the original driveline, just in case the next owner wants it.