Yes, no, maybe to all of the above.
1. Yes, The '55-56 Packard V8 and the last big block Cadillac 472"-500" were the only US production car V8s with 5" bore spacing. The '56 Caribbean with 374", 2x4-bbls, 10.5 compression, was the largest displacement and second most powerful engine available in that year.
The Packard V8 was a mix of the GM 1949 Gen I Olds and Cad V8s, having an extended bell housing, shaft rocker arms, air-gap intake manifold and siamesed center exhaust ports. It has the head intake and exhaust surfaces at 90-degrees to the block face and unsupported lifter bores, similar to the Gen 1.5 Pontiac V8s.
2. No, Packard was out of business before their new V8 was ever de-bugged. There was never any plan or tooling for a V12 version. The V8 engine is 30" long and weighs 700# complete. That would make a V12 more than 40" long and weighing more than 1000#. Think of any modern car engine with those dimensions?
3. Yes, Chevrolet Engineering did a Mark III feasibility study of buying the tooling for their big block. They decided not to go there because:
a. Not Invented Here. Engineers usually get only one clean sheet engine design opportunity in their careers. Who'd want to give that up and de-bug an already designed engine?
b. Didn't want the brand tainted by bringing in a failed car company design.
4. Yes, the Mark II '63 Daytona Mystery motor used the same basic big block head design we have today, just installed on a square deck version of the Mark I Z11 W-motor.
5. Yes, the Packard V8s 5" bore spacing would have allowed an easy 500"; same as the big Cad. It would have needed further development, including stronger main bearing webs, reinforced lifter bores lifter bores and an improved oiling system.
6. And no, but someones sure to ask, the Rolls-Royce V8 was a clean sheet and shared no parts, dimensions or design features with the Packard V8.
Studebaker-Packard V8 Limited