so I'm looking at a 1983 Buick regal and it came from the factory with a 4.3 diesel but that's long gone so my question is what would be my option for an engine swap would I be able to run an olds or Buick 350 without switching mounts or would I have to switch them regardless

 

Assuming that is the 260 V8 diesel and not teh 263 V6 diesel, any 1964-1990 Olds V8 bolts in. Note that diesel cars offset the driver side motor mount and frame pad forward to make room for the diesel starter. There was an adapter plate bolted to the block to provide an attachment point for the motor mount. This photo shows the diesel location for the frame mount on the right. For gas engines they are directly across from each other. You can see that the holes are already drilled in the frame for the gas engine location of the frame pad.
You can transfer the adapter plate for the motor mount to the gas block, but that motor mount location might get in the way of headers if you plan to use them.

 

what would be the case if it was the 263

 

Not sure, as this photo doesn't show the driver's side. The passenger side mount appears to be in the same location as for a V8.