Antifreeze in cylinders in 78' Merc 454

Morning All, Here's the skinny...... 1978 26' Tollycraft with Merc. CBB 454 was 330 hp, rebuilt 6 yrs ago to 425 hp, all roller rocker motor now and has ran perfectly since. Three years ago we purchased a larger boat so the Tolly has been sitting. At the end of the last season we used the boat the fresh water pump went out and we over heated. I replaced the pump, ran the engine and everything seamed to be fine, I discussed the issue with a marine mechanic and we assumed that I didn't blown a head gasket. We used the boat another 1/2 doz. times than put it away. Jump ahead now 3 yrs.

Yesterday I went to start it and it was Hydro-Locked. Pulled the plugs and had antifreeze in #3 & #5. The engine rolled over just fine, but the cylinders never cleared out, they just kept spitting more antifreeze out. (The Exhaust Manifolds are fresh water cooled.) SO.... the Questions is 1. Do I have a Blown Head Gasket. 2. Do I have a Blown Intake Manifold Gasket. 3. Do I have a crack in the exhaust Manifold Or a combination of the 3 ?????

Thanks, Jesse

1 Blown head gasket between 3 and 5 the 2 center ports
2 unlikely, no water/AF passage in that area
3 depends on the severity of the overheat and if the A/F had proper freeze protection