Does anyone know anyone or hear of a 600ci tall deck big block in a 3rd gen?

A tall deck big block is EXTREMELY difficult to get into one of these cars; especially while keeping power brakes and the AC. But like almost anything, with enough money, time, patience, hacking, and acetylene, it can be done.

How many cubic inches it has isn't an issue. If you can get a 600 inch motor, it'll be exactly as easy or hard to fit, as a 366.

A 540 is a 4.5" bore and a 4.250" stroke.
A 572 is a 4.5" bore and a 4.5" stroke.
A 632 which is the common large size uses a 4.600" bore and a 4.75" stroke.

My 540 is only a short deck block. I can bore it out to 565. Although i could put a 4.5" crank into it for more displacement, the short deck block tends to put the wrist pin even higher into the piston so the 4.5" and up cranks are common for the tall deck blocks or the super tall deck blocks.

As mentioned above, tall deck blocks create a lot of other problems. With the deck surface .400" higher, it moves the heads up and away. Using a good performance heads, at least some 345's for a big engine, means they will have raised exhaust ports which will cause even more problems. No off the shelf headers will fit so you would need custom built, big tube, headers for such a big engine. Headers alone would be a minimum of $1500. Normally engines that big don't use common BBC heads. They'll use Big Chief heads which uses different intake and exhaust manifolds. (not cheap)

The worst clearance issues would be the wiper motor and heater/AC if it's in a street car. Depending on the intake used, hood clearance may also be an issue.

Nobody uses the 366 tall deck truck block. The bore is too small for any performance potential. The 427 tall deck block is still used but now with the availability of aftermarket 4.5" bore blocks, the old truck blocks are not used as much as they once were.

You could easily install a 632 if you had a complete tube chassis car.