[LS6 RAT]

I don't agree with your assertion of 15 to 20% of original production is all that is left. The survivor rate percentages are way different, depending on the type of vehicle. I would say '69 Z28's or '70 Boss Mustang 302's have a higher percentage of attrition due to the nature of their intended usage, however the more exotic musclecars that had higher price tags when new often were cared for by their owners and thus survived the ravages of time and usage much better.
An example would be the 1971 LS6 Corvette, total production of 188 examples, 12 in racing trim. These cars had an average price that exceeded $7000.00 when new, more closer to $7,418.00. Survivor rate currently is at damn near 50%, and may end up even higher. I've been involved with these cars for over 20years now. I have located 92 of the 188 production and expect to find a few more before I die.
I don't think a generalization of musclecar survival rates percentages can be so easily tossed out there.