actually the LS5 454 is a little more rare than the LS6. of course the LS6 is more desirable and worth more money than the LS5 due to performance comparison. the production number for a 1970 Chevelle with a LS6 is 4,475. the LS5 is 4,298.
i have a set of LS6 heads on my 70 Chevelle (originally an LS5 car). those heads alone are worth BIG $$$$. stock they are HI-PERF closed chamber, rectangular port, with 2.19 intake valves and 1.88 exhaust. being is that they are closed chamber heads, and have small 109cc combustion chambers they can give you high compression ratios easily. when you pull the valve covers off, you will see HI-PERF, and the casting number is 3964291. if it doesnt say that on the heads, then they are not LS6 heads.