I saw someone some time back contemplating putting some sort of strait 6 in one. I recall the person had measurements that suggested that it could fit. Others seemed to disagree.
People obviously put 8 cylinders in BII and I believe to get them in there needs to be some body lift to get space for bell housing/larger trans or other mods to the firewall to accommodate such.
What strikes me as a challenge is that inline 6s are loooong. V8 tend to be more compact length wise with just 4 cylinders length plus some verses 6 cylinders in a line.
Now compared to V8 which generally have significantly more power/torque than the standard 2.8L/2.9L used in BIIs forcing upgrade of driveline components, a strait 6 in the ball park may be okay with the stock hardware but getting it to mate up I don't know how one does that.