How complicated would it be to swap a stock v-6 with a police intercepter 305?

 

Quite, actually. Typically makes more sense economically to sell the V6 car and buy a factory V8 car, even with a bad engine. The V6 car has to have a lot of sentimental value (like being your first car or one purchased new by your family) to make to worthwhile.

The best approach if you go ahead with the swap is to get a donor factory V8 car (close to or same year & model as yours) to get all the parts you'll need - transmission, mounts, springs, accessories, radiator, harnesses, ECM, exhaust, tach, etc., etc., etc. Set the two cars side-by-side, take pictures of both before starting to disassemble, label everything on both cars as you take them apart.

The engine mounts on the frame are easiest to move while the a-arms are off, which you have to remove to switch the springs, but that's easiest to do with the weight of the engine to help compress the spring - a Catch-22 that's an example of the obstacles you will encounter.

It can be done (obviously), but don't expect it to pay off financially.