That is the 290 HP 318 that came in 57 Furys and was optional in all the other models. I had that motor back in 1964 when I got my car. I added an Isky cam and Hedman headers and it ran great.
57 Belvederes came with a 301 2 barrel with an upgrade available to a 4 barrel and duals. The Fury V-800 318 with 2-4 barrels and 290 HP was also available. I'm betting this one has a 301 2 barrel in it.
The rockers are available and not that much money. The eyebrow pieces are also available. I still think that if the right people see it, you would get some takers. If it was a coupe it would go for a lot of money in that condition.