It's an '83 Fleetwood Brougham d'Elegance. Right now it has the original HT4100 in it, too underpowered and simply too small for my taste. I was wondering if anybody knows, for sure, if a 500 would fit in the car.

 

Are you talkin about a GM crate motor 502 , 500hp?

 

Don't forget, the 500ci needs a TH-400 tranny.
Physically it will fit but it won't be a simple, hassle-free swap. Up until 1981 (368ci) the engines belonged in the same family, not so after that. You'll have to fabricate and massage lots of things and be generally innovative to do the job right.