Yep, that's an '81 or so header clip on the front of that car. Nothing wrong with that, but if it's an '87, I'd put the correct header panel on it as well.
I like the 3800 series motors just fine (we owned a 2000 Regal GS that I loved dearly), but I'd never put one in a full size 80's Regal.
Just wouldn't do it......but that's just me.
Good luck with the build, I'm sure it will turn out very nice.