I'd have to check, but I believe the powertrain on the '84 is a longitudinal engine with a trans that is a derivative of the 200r4.
The 4.x FWD engines are transverse with a transverse tranny.
I suspect, if I'm right about the engines, that the subframe is longitudinal as well with "standard" rwd-type motor mounts.
If emissions weren't a consideration, I would think shoehorning in a 472 or 500CID eldorado powertrain (with the TH400-based transaxle) (which was also longitudinal) would be a fun project. I imagine the fabrication involved to get the transverse 4.x powertrain into the 4100 longitudinal engine bay would be pretty extensive and not give any real benefit.
Does the '84 have a strut-based (no upper control arms) or shock-based front suspension? Either way, its no small project.
I don't know much about those old powertrains. Does the trans bolt pattern fit other GM engines? Perhaps a 350 chevy could fit in, maybe something like an LT1. Doubt the trans would last long, but that's the "fun" of GM FWD transaxles in general.