not likely iron headed motors rarely blow head gaskets they have to be really really over heated. Normal vacuum for a stock engine is about 22 in so you are way low.....do a compression test and post the results.
Take care, Rick 68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451
