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I would not go 15 ft pounds!!!
Just snug them up .
Install the bolts ,take out the slack and with a socket just turn them snug.
Do not tighten them and tighten them over and over you will break one or more off.
15 ft. lbs. is too much. GM Shop Manual says only 8 ft. lbs. for the transmission pan bolts. I'll trust the GM Shop Manual any day over the Chilton's. I don't like the Chilton's. Specially when it tells you that you have to drop the whole gas tank to remove the fuel pump assembly.
8 lb/ft would be 96 lb/in which agrees with Haynes 97 lb/in (with-in torque wrench tolerances). It is sometimes a good idea to cross referrence service manuals because there are typo and other errors in manuals. Also keep your own common sense on alert and when something does not seem right stop and get a "for sure" answer before going further. Gordon Killebrew (retired from GM; former C4 development engineer) told us about errors that are in the GM factory Corvette service manuals.