I have a 1993 Bonneville with a good running 3800 and tranny.
I just picked up a 1996 Bonneville with a blown 3800 as a donor body.

How much of the 93 engine, harness etc will I need to rip out and swap, or is it a simple plug & play?

You would need to swap over everything from the 93 to the 96 because just about all of the sensors changed to the new OBD 2 standard in 96. If your vehicle needs to be plugged in for any type of inspection, it would probably fail because of the mismatch in years.

In a nutshell, the 93 PCM would not work in the 96 body because some wires/harness'* change.

Thanks for the input.

The '96 threw a con rod, but top end is all still good.

How close / different are the engine blocks and crankshaft between the '93 and '96?
Could I possibly use the core block from the old '93 and bolt on all the useable ver II stuff, sensors etc?

No. All the internals are slightly different, including the external parts.

Wiki gives a brief description in the Series 2 section.

Buick V6 engine - Wikipedia, the free encyclopedia

If I had a Series 1 service manual, I could give you all the sizes of bearings etc.

This artacal has a ton of info on the 3.8 that I found informative.
Rebuilding the 3.8L Buick Engine: Engine Builder

It will probably be much easier to sell the '93 and find a Series2/3 block to swap.

I done what you want to do but in reverse. you have to change all the abs, engine, dash, cab wiring harness, pretty much everything electrical except stuff like fuel pump harness, and taillight harness and stuff like that.

I quickly realized the extent of the work required to swap out all dash wiring etc.
Seemed like a lot of work for a step backwards in engines.

So, with the help of Wikipedia telling me the exact vehicles and years with the 3.8 Series II, I headed to the local Pick N Pull.
I lucked out and found a 1996 Buick Park Avenue Ultra with a Supercharged engine in apparently decent shape. 8 hours later, we walked out with a wheelbarrow full of engine.

Turned out to be a simple plug and play and ran perfectly without a hitch.

Thanks for the help and the opinions here.
I'll second the suggestion that swapping a series I 3.8 into a vehicle that had a Series II would be a heck of a lot of work, as you need to swap out not only the engine bay wiring harness, but the entire under dash and console wiring. A LOT of work, but do-able.

ohh yes doable but lots of work. blame it all on obd2..lol