I think the 350 in a low mileage decent unmolested example would still fetch a decent price one day, opposed to the 370 as it has not been to everybody's liking.   A car in future years  to come, depends purely to supply and demand with as said before an original low mileage example being top of the tree.  The 240/260z became the sought after models,	where as their later replacements   ie  280zx  300zx have never had the same attraction.