What do you mean by Big Block ? To me its a Mark IV 396 to 454 Chev power. 

Good Question Ray.   Not sure what the engine options were for a 1981 Firebird. Saw the car on Kijiji this morning thought I'd ask a few questions of the membership prior to engaging the owner.  

1981 Firebird Engines

So no Big Blocks were factory available in 1981 Firebirds, Although a Pontiac 400 or 455 would easily swap in and these are sometimes referred to as Big Blocks .

Todd's idea is a more expensive but good option. Since the Parisienne has Chevy small block power there are a lot of straightforward swaps available to you. 

Crosstown engines here in Toronto sells lots of reman engines, I'm partial to the Vortec 5.7 Code R 255 HP 365 lb ft. It is the last of the true original small blocks. GM sells this with a few modifications as as the Ram Jet crate engine.

Here's the listing:

73SC wrote:

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Double D wrote:

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www.kijiji.ca/v-classic-cars/chatham-kent/price-drop-1982-big-block-firebird/1115565161

Take a look!

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 That's an Olds engine. I guess its a 455. Funnily enough US Bonnevilles got the 403 Olds in that era of car but I am not familar enough to give any advice on swapping that into the Parisenne.

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They put the 403 Olds in the Firebirds in that era as GM stopped making Pontiac blocks.    If you're doing any type of swap, I'd go with a Small Block Chevrolet 350.

Fewest headaches and the most straighforward swap would be with another small block Chevy. Be aware that the 267 probably has a TH-200 behind it; you will want something stronger like a TH-350. Also a 1981 probably has a lockup converter; the earliest versions were known for being a bit rough and clunky. A 700R4 or 200R4 would be a really nice upgrade, but you would want to change out you rear gearing since it probably has a 2.41 or something similar already. Maybe a 3.42 with a 4-speed o.d. would be sweet. Or you could keep it simple and pick up a straight forward TH-350, forget the rear swap and drop in a 350 V8.

I just thought of something, some of those 197n big cars used a puny 7.5" rear end (typically 267 & 305 cars). They can't handle much, so balance your drivetrain options carefully.

When all is said and done, it seems like a lot of work to go through for a '81 Parisienne. Just my opinion.