Jason,
You are correct. I have both an LS-6 & an LS-7 that were purchased from GM in the late 1980's. The LS-6 has GM Part #366250 and had a engine suffix of "XAA" and the LS-7 has GM Part# 3965774 and had a engine suffix of "XCH".
I found out the hard way you could run the LS-6 like you stole it right out of the crate, but the LS-7 had poor tolerances, and spun a main @ 100 miles. When we took it apart, 1 cylinder was over by .010 then any other, so it had to be bored .030 and it has never been fired since(that's the engine I now have for sale in the classified section under "Big Block Garage Sale"
His TD13XAA is probably TO13XAA for January 3rd 198? XAA an LS-6 Complete Crate Engine
Bill