Probably a failed head gasket or cracked head. Just drain the milkshake out and leave the pan under it for a while it'll mostly come out. You must have really got it hot, as it take a lot to get an iron headed motor to fail a head gasket.

Take care,
Rick
68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451