Not true, there are several companies making sleeve kits, cams, pistons and rods. There is even a company making new heads. If your block is ok, you can build one for around the 8-10K range with all new upgraded components. (less depending on what all is bad) Or buy a crate engine with the same...
The biggest question is why? With several synthetics that have more than proven themselves over the years, what will it do over the others? Amsoil, Royal Purple, Redline, and of course Mobil 1 just to name a few, would be my choice unless there is something specific in Schaeffer's that the 6.4...