In all the digging I did leading up to my swap (which you referenced in the first link above), I seemed to see a lot of mixed information, but it made me fairly optimistic that the heater box modifications could/would be minimal. Obviously in my case that wasn’t true. Another common train of thought was that swaps to larger engines (even BBC) using the non-A/C heater box required no modifications to the box. And again in my case, this wasn’t true.
I’m pretty confident my experience was a “worst case scenario”, but my advice would be to check the fitment/clearance before you start cutting away at the heater box. In many ways the 3.8 is just “a Buick 350 with 2 less cylinders”, but in reality the 350 has a taller deck height and is noticeably wider, which can make this swap a bit tricky.
Here’s a shot from above showing how close the valve cover would’ve been to the heater core inside the A/C heater box.
This picture shows the interference I had between the 350 valve cover and the non-A/C heater box.
This is a rough mockup I ended up with after modifying the non-A/C box so the valve cover could be removed for future maintenance purposes.
This is what I ended up with. Even with removing a sizable section of the non-A/C box I can just barely get the valve cover off the engine.