It looks like the rear axle could be original. 3.31 was the standard ratio for SS350 cars and 396/350hp cars. It was the performance ratio for 396/325hp, and economy ratio for 396/375hp cars. Just looking at the production numbers, the SS350 is the most common.
The reason I asked about the cross member is that big blocks use a different one than small block cars. The more big block components that it has (radiator, heater box, cross member, etc.) the better chance we have of figuring out what engine it had originally. Since transmission case is a replacement, that doesn't help.
If the paint is good and just from the general appearance of the pics posted, I would say at least $30K and maybe more. Car looks good and is desirable. I don't know what price you were thinking, but you could start at $40K and see what happens if that is close to what you were thinking. Remember as the seller - you can always negotiate the price down, but not up.
If it could be proven as an L78 car, with docs or other methods, it would be worth more than that. And if it was the original color, maybe quite a bit more than that.
Good luck.