Although CRR suffix was originally used on block #3963512 for a 1970 LS-6 Chevelle with an TH400 on the assembly line,I see your engine block as not being exactly the same.
Your block number #3999289 is a 2-bolt block & casted Jan.14th,1972,(Real LS-6 was a 4-bolt),has subtle design differences,and was a 1972-75 Pass-Chevelle-Corvette-Monte Carlo application.
Had anywhere from 215-270HP.
I see this as a truck application really,as that is what it was used in mostly from 1972 through 1978.
I would just be happy with it if you enjoy it,as that's all that matters.
