What do those numbers mean? I'm a first time buyer of a Chevelle and I don't know what they mean! Can somebody PLEASE tell me what they mean for each one?

the LS# is the RPO code that relates to the engine size. LS-7 is a 454/? that was a over the counter motor LS5 is a 454/390 depends which year. and so on

LS-7 = Over the counter 12.25:1 compression 454, rated at 460-475hp. depending on who you talk to. Discontinued in the '90's., real horsepower probably around 560-575. An over the counter race engine, .560-.600 lift solid cam, duration of @ 260-270 at .050 lift, rectangle port heads, coded XCH Old part #3965774 for a long block, short block #N/A. Not available - I've looked!

LS-5 = Factory installed 454-360(390) hp. depending on car application. Corvette had a free flowing exhaust manifold, and was rated 30 hp higher.

LS-3 = Factory installed 402-330hp., or less, depending on year. Net vs. Gross hp. change explains the decrease in horsepower, as the power was the virtually the same, just the measurement style was different.

Along those lines everything is all RPO (Regular Production Option) codes. Just like Z16, Z/28, ZL1, ZR1. Some of those arent really what we would call regular because of the rarity, Etc. But its just like mentioned above.

They are still using this marketing or whatever you call it today. Like Z71 package on GM trucks. Im not totally sure but I think the Corvette ZO6 is the number based on the option sheet. Im not rich enough to shop and find out for sure.

Im not totally sure but I think the Corvette ZO6 is the number based on the option sheet.
yes just like in 63