I just bought my c5 FRC about 4 months ago. I came from 7 years of building mustangs and got tired of it. Now im learning about the ls motors. heres my dumb question.
So, from what I understand, ls1/ls6 heads are not interchangeable with ls2/3/7. Then why do people lisy 799 heads witch are gen 4 the same as 243's whitch are gen 3? Is there a thread on what is interchangeable between motors?

 

243s and 799 are the same LS6 style head. The 799 casting number is just used on the newer LS6 heads that came on some LS2s and some 5.3l motors. 243 and 799 heads will technically fit on all of those motors they just won't perform as well on the bigger motors. LS3 heads require a 4.0" bore or larger so they will not fit on an LS1/LS6 motor but they will fit on LS2/LQ9/LQ4 and larger. LS7 heads require a 4.125" bore or larger so they really only fit on LS7 motors or an aftermarket motor with a large bore.