I watched this last night and enjoyed it. It's great to see people doing engine conversions on these rare and valuable skylines without regard for future value.
Reality check: C10-series Skyline production volume for 1968 through 1972 = 460,000-ish. And still plenty of them left.
The "rare and valuable" C10-series Skylines are the PGC10 and KPGC10 GT-Rs, with around 2,000 made.
What you're seeing is people modifying lesser models to look like premium models, and engine-swapping for performance gains (same as happens on S30-series Zs). Most of the people commissioning these modifications would appear to be fairly sure that they are adding value to their cars, as that's where the market *is*...