Your local GM dealer might be able to get some parts,but I suspect that some are discontinued peices,such as the heads.Your LS-5 block should be a 4 bolt main and it should have a forged steel crank,although it might not have the HD rods with 7/16 rod bolts.The LS-6 did not come with an engine oil cooler.The correct heads are casting # 3964291(I think) they are the closed chamber rectangular port design made from 1969-1970,probably pretty hard to find.Rods are part # 39635552,bolts are # 14096148 and nut # 14044866,this is the 12 pt 4037 steel nut.I've got 3 part #'s for the cam,12364057 may be a current #,39044362 & 3863148,lifter is part # 5232695.Intake is a low rise big port aluminum model # 3977609,there is a Holley carb with a GM part # to go with it but I cant find it.Pistons wrere made by TRW for GM(I think) and are GM part # 3976013.Some of these part #'s are old ones and may have been replaced by newer #'s.Theres probably a lot more than these to be 100 % correct,but you get the idea.You might get lucky and find someone with all the parts you need stashed away in the corner somewheres,but I dont think that you will find any vendors with a complete package of parts like you want.Try a Chevy dealer with a parts guy or mechanic who has been around since the early seventies or an old drag racer.Let me know if this is what you are looking for,I may have another book with more part #'s.
Guy
Guy
