I would not go with the 2.8 out of a firebird at any rate....They were either OK or REALLY bad.
I would suggest a buick 231 v-6 (also called the 3.8l) They were used to power full sized Le-sabres and Centuries in the 70's and actually did so pretty well. Additionally, if you get a BOP transmission in the caddy, it would bolt in.
I had a '76 Century 4 door with the 231 and got 18 around town and 30 on the highway. This was WAY before I had ever thought about worrying about fuel economy or modding a vehicle for it...so it was mostly stock. It was a great car until the tranny went out. A 60's Caddy would be a little heavier....but likely still driveable.
The motor is known for it's longevity as well...and if you really want to get silly with power, it is the same motor that was used in the 80's for the Grand National with the turbo
Jim