To add to my last post. back in the mid 70's, I had a 326 Firebird...I put on a 650 CFM Holley and it worked great...Well as great as a 326 can work...LOL...I dropped in a 400 with a bit of head work, cam, lifters, Offenhauser duel port intake and a set of Appliance headers (has anyone else ever heard of Appliance Headers?), a few other minor mods and kept the 650 Holley...When I walked on it, I think I had more power with my little 326. I went to a 850 Holley double pumper and it woke everything up.
That's why I thought he may be under carburated.