Why do you want to change the motor to a chevy.
These old caddy motors were state of the art domestic engine design of the day. Remember SBC's were designed at the same time and designed to be cheaper to build.
You'd be taking a step down for a lot of bother. What's wrong with the caddy motor that can't be fixed. Sell off all the chevy parts ya got laying around to help finance rebuilding or repairing the caddy motor as required.
I'll bet what ever is wrong with your caddy motor can be fixed for less $$$ overall than swapping in a SBC. You can even do some modernizing like electronic ignition to eliminate the old Delco points.