I don’t think which weight you choose changes it much. Carpentier would have a punchers chance with that right hand of his, but I expect Fitz would be much too smart and shifty and batter Carpentier in 6 or 7 rounds.
Fitz was just a level above IMO. Unless you mean the version that actually won the Light Heavyweight title, who was a total mess that fight, even then I think he'd hold his own.
That's the only weight ,light heavy ,where he'd stand a chance of winning .As u point out Fitz wasn't at his best and ageing fast .At middle and Heavyweight I've got Bob by ko ,unsure about Light heavy.
I looked at some reports of it in another thread. I think it was Mendoza's one on if Fitz's record was better before or after Corbett. IIRC he injured his hand fighting Jeffries and reinjured it trying to KO Grim a few weeks before fighting Gardner. I think he also put on a fair bit of weight, and drained himself cutting back down. He reported felling ill and collapsed with pneumonia soon after. Considering the issues he had, I think there's a case for it being his most impressive performance. But you'd need to check the thread, reports etc. as it was a while ago
I believe I would take Fitz at Middle and Heavy,at Lhvy he was an old man ,unless you want to consider him in the frame earlier when he comfortably made the weight ,in which case it's a clean sweep for Fitz,imo.