I had a '77 GP, too. Dark silver/gray with black leather interior, and T-tops. Fun car! But I digress. I never though about the "smaller target", as I believe the law here in Washington says they have to have covers on them when not in use off road (that is to say, must be covered when being driven on public roads). I'm not absolutely certain that's true, but it's what I've been told for years. Also, I like round lights better, though KC does make some round LED lights, as well. Thus far, my only experience with LED headlights was on my Mazda, and I loathed them. $100 for two "bulbs", and I swear my eyes never got used to them. I'd be afraid to spend the coin on some off road lights, only to hate them.yes... I did this on my 1977 Grand Prix... when it was new. I put 100 watt halogens in the high beam location and had to add the relay breaker to handle the load off the high beam foot switch. The wiring had no issues.
Today you likely need to pull in new wires too. But using the fog switch on the multi function switch should be easy. Today I'd buy the KC harness for a start.
On the LED thing, I was a hater too. I was going to go with Warn halogens on the A Pillars but for simplicity, load, $$$ I went with 2 Kawell Side Shooter, $69.00. And I have a single LED bar for the roof. They all run thru the factory fog wiring. Took years to take the jump. Glad I did. They are a much stronger, smaller target for winter sand, salt.